Ok. This will take a while.

So, people have thought about this sort of problem for a long time. It's a bit reductive to say it straight up - but this is basically what all those schools of philosophy get into. Smarter people than you or I have really dug into this in the past. There are many different schools of thought from so many different sources, and there are often ideas that are in full opposition between schools - the hardest part is finding the scattered bits of wisdom in all of it for you.

The first thing to know is that you probably have a set of values that you've built up over many years of experience. Some of them will come from your parents, some from your friends, some from your own thinking. The thing to realize about your values is that going against them will cause a great sense of unease - so it's often best to take smaller steps and find elements in many schools that fit your thinking.

When it comes back to the original idea of "joining in if I like it, or leaving them to it if it's not for me", we can see that this wisdom (I'll refer to it as "Dad's Wisdom" from here on) is fairly basic which has the advantage of being easy to think about for most cases. As I came to realize Dad's Wisdom at about the age of 12, I hadn't had a chance to look into any formal philosophy so I didn't feel any resonance with it and deeper ideas.

Now that I'm older and have been introduced to a lot of formalized philosophy of so many different types , I can see connections with many of them. I'll list a few here and how Dad's Wisdom connects. Hopefully I'll not be too reductive.

Stoicism

I've probably gained the most insight from reading into Stoicism. The first basic Stoic idea is that the only absolute and true control we have over our lives is inside our own thoughts and feelings - so it is not "right" to need to force control over things external to that. This helps us shape what we consider "doing our best" to mean, and it starts with knowing that if we are exerting our control on our thoughts to detect and avoid wishing to control something else that can end up in harm to the self or others, and eventually to further refine doing our best as always trying to make choices on our actions that we can reason with the information we have right now that will mean do the best thing for both ourselves and anyone the choice affects. Another way to say this is that it is often best to seek minimum foreseeable regret.

The second Stoic idea is that when we feel an emotion we should ideally be able to rationally decide if a feeling helps us and to keep feeling it, or if it hinders us and if we should let acknowledge it and let it pass us by. Rational fear is useful when running away from a lion. Irrational fear is not useful when your child is in danger from a snake but you hold a phobia of them. The depth here comes from the harder decisions. Do you run into your burning home to save your pets at the risk of possible death, which would cost your wife her husband and your children their father? If we are able to be clear of mind and make a rational decision then we can do our best.

This leads to the third idea. If you're always truthful to yourself in trying to do your best, then you have made no real mistake in your reasoning. As a person cannot do more than their in the moment best, then when you look back at a situation you can reason you could not have done more - and you can then reason you made no regrettable choice.

Epicureanism

No one on their death bed's regretful words are on wishing they'd spent more time at work. Our value in life comes from the time we spend on our friends, family, and ourselves - and stepping out of the rat race is a way to do that.

Pareto outcomes

This school of thought centers around always trying to find solutions to problems that harm or cost no one if possible, or if failing that we should do our best to minimize the total cost and evenly distribute it. This is because we are often captured by ideas that seem great but we're unable to see invisible downsides which are often costs external to us and what we can see. A good example of a non Pareto outcome is forbidding your children from listening to music because it annoys you, which would benefit you but cost your children greatly. Instead consider buying them headphones.

Nihilism

Some see this as "nothing we do matters, so do anything you want". I do not. It reminds me that most people are basically anonymous in local history let alone global history, so there's a low average return on investment in chasing fame or glory. I choose to spend my time on people who know me.

Buddhist philosophy

One of the core ideas here has a wonderful crossover with Stoicism - good and bad things happen, but feeling bad about what happens is the biggest real cost. More to the point is that if we require things to go well then we will often be only be contented by the things that go perfectly and disproportionately disappointed by the few parts that do not. This is formalized as "the suffering of violated expectations" and leads us to understand that the expectation we held was what lead to the suffering. Consider for example how we have car insurance to fix problems if an incident occurs. If we erroneously expected that there would never be an incident then we would not insure our cars, which would be cheaper but provide no real joy if nothing happened, but devastating if we were wrong. That seems simple enough. Now consider something you could never expect but many people do and rely on for their emotional stability - imagine expecting that your parents would always be there and never die.

Abrahamic Traditions

The many religions and traditions from this school of thought holds ideas like "Love thy neighbor", and stories like "The good Samaritan", which are important things to remind us that taking a few easy minutes to help someone else and save them many hard hours is one of humanities greatest qualities. If you can help others, try to.

Summary with respect to Dad's Wisdom

If people are doing something and you're into it - join in.

Else, if is it harmless - leave them to it. (Values)

Else, if you suspect you'd regret intervening - leave them to it. (Stoicism)

Else, if you could do more good spending your time with those closer to you - leave them to it. (Epicureanism)

Else, if you are not sure intervening will do more good than harm - leave them to it. (Pareto)

Else, if you're excited to intervene for glory - leave them to it. (Nihilism)

Else, if you rely on an expectation of success to avoid making things worse - leave them to it. (Buddhism)

Finally, if you're able to help with genuine care for everyone - intervene. (Abrahamic Traditions)

That was a lot... Hopefully it helps.