If your friend is genuinely unattractive he may need to persist more than the average guy. He has gotten laid by sacrificing his dignity, integrity just for sex.
Men can pursue, up to a limit. The issue is when you are pursuing and the lady isn't reciprocating and you're still pursing to change her mind.
If you went on 4 or 5 dates, it means the woman was interested, at least initially. It's okay to pursue interested women! In those 4 or 5 dates the woman has changed her mind so its something you've done to make her feel different about you. Also as you get older a ladies priorities change, they are looking to settle, get married, have children etc, so you can't employ the same game.
I know what you mean, you need to get better at assessing her true interest. This is part of your self development.