While I agree with the "become the best version of yourself that you can be" part, I've always had a more simplified and practical approach to the nice-guy predicament. Specifically, even though society has been progressing in a manner that increasingly prioritizes brain over brawn, Darwinian selection still applies, and the fact remains that "nice" is the modern equivalent of weak.

No one (man or woman) wants to be with someone who can be pushed over and easily manipulated, because there are assholes in the world who will inevitably try to push you around, from a racist clerk at the DMV, to idiots who will cut into the line at the theater, or inconsiderate pricks who take up free seats on public transportation with backpacks or feet. Being a nice guy in these situations not only reflects poorly on you, but also on your partner, and gives them the signal that they may not have emotional support when heavy lifting needs to be done in social situations.

If the mechanic over-charges for an oil-change, stand up for yourself. If a neighbor's dog is shitting in your yard, say something. If the waiter brings a shrimp cocktail, be polite, but don't just sit there when you know you didn't order it because you're allergic to shellfish. You don't have to be an asshole, you don't have to be a tough person, you don't have to be mean. You can be a good person, you can be a decent person, you can be a funny person, you can even be a quiet person, but don't be a nice person.