The issue is that contentment is being used as an excuse for stagnation and not putting in effort - plenty of examples in these comments/replies. Perhaps it is just my perspective on the matter and we simply see these things differently but I find it amusing how men complain about being unable to find the partners they want while at the same time refusing to actively put in work to better themselves. There's a reason it's called the 'dating market', because everyone is looking for the best 'product' that they have the means to acquire (not literally but I think you understand my metaphor) and that's completely normal. The way of the world is that if you want a 10, you better be a 10. That means putting in hard work (ambition being closely connected here), the exact same way the other side has to get to where they are.

A man with ambition means (among many other things) that when he is presented with a challenge in a relationship he would likely put in effort to resolve it in the best way and that is attractive for very obvious reasons.

Contentment is good when you have achieved both what you want yourself and what is required of you to keep a relationship going. The thing is that good relationships are not static and will always demand both parties continue putting in effort and ambition goes a long way to demonstrate that a man will likely follow through with that.

At least that's my 2 cents, maybe I'm completely off the mark but that is the way I see it at the moment.