Why people say stop looking for love and let find you?

I think that this advice applies up to a point.

It is true that if you're just looking to be in a relationship, ANY relationship, that's foolish; being in a relationship is not necessarily better than being single in and of itself. Ask anyone who has ever been in an abusive relationship and tried to leave it. Even in less extreme situations, if it's not working out, you're going to get resentful and wish you were free to date other people.

As for why it doesn't work for finding people, it's because desperation and insecurity aren't attractive, and if you have enough of that it's going to rub off regardless of whether you intend it or not.

However, if you are interested in a specific person, you can't just expect that things will naturally develop. Sometimes they do, but usually they involve at least a little bit of work from both parties - finding some way to make your feelings known (it doesn't have to be OBVIOUS and that's often a turn-off anyway, but you should make it clear in some way or another), initiating dates, etc. Hanging around someone just waiting for an attraction to develop does not work.

Another thing I'd like to add: I'm not saying this is the case with you, but it seems to be the case with a lot of guys (more so than girls) who are perennially dateless that they think they that they can put zero effort into their personal appearance because they know they are awesome people inside. First impressions are important, though, and if it's bad, people aren't going to stick around to find out about your deeper qualities. When someone's already your girlfriend you can show your warts-and-all self but not on a first meeting. And I'm not just talking about how you dress and your personal hygiene, but also to be more polite. Put your best face forward. It's not about trying to be someone you're not (which is always bad), it's about being an enhanced version of your normal self. Be you but just a little better.

Also would like to echo that just trying to meet more people, going to places where people have similar interests to you is a good idea, although again, you can't just expect that these friendships will magically turn into relationships if you don't put some effort in that direction. If you want a place where you're sure that people are looking for romance or sex, you could go to a singles bar or an online dating site, but that does add an extra level of awkwardness that isn't there when you start out by approaching someone as a friend and based on similar interests rather than just a shared desire to not be single anymore.

TL;DR: Desperately looking for a relationship is bad, but once you have someone you're attracted to, you can't just wait for it to fall into your lap. You have to do something about it.