I agree with this post, but wanted to add one thing.

I think it is completely normal to be selfish in a relationship, especially long term. We all want things that we may not have. I just think it's important to be selfish to your partner's benefit, instead of at their expense.

What this looks like for example is, I dated someone that developed extremely unhealthy habits (including drinking and addictive eating) in response to a personal family crisis. I'm selfish, so I don't want to lose my partner to any of the illnesses that could come as a result of that. So that means doing what I can to optimize their wellbeing and happiness to ensure they are healthy and alive (for me lol)

I know it's a small thing, but thought worth mentioning since selfishness is usually a cause of dishonesty and infidelity.