There's (not always, but) often a tangible point where my acquaintances tend to turn into friends. It's usually some bonding experience. We stayed up all night and did X. We drank together and had a heart to heart. We both love doing Y and spent a lot of time doing that. We travelled together. We suffered through college classes. We spent weekends hanging out together. We spent years talking on IRC. Often there's also some something about level of trust. Someone reveals something about themselves. They talk about something personal that they normally wouldn't. Etc. Absent either of those, my acquaintances tend to stay acquaintances.
On the flipside, I've found that keeping around people who don't value you, don't have your best interests at heart or intentionally cause you to feel bad in any way is just a bad idea. Wanting to become better friends doesn't make it so. It's a two sided thing. Really, I love most of my friends and I'd sacrifice much for them, so when it comes to people like this, I don't have time for that shit.
I have a relatively small number of close friends, and I don't get to spend time with all of them all the time. And as time passes and life happens, friendships form and fade (and I find that the good ones can kickstart right back - even years later.) All that is fine and natural.
It's really about mutual happiness, fun and sharing, and the rest doesn't matter. It doesn't have to be complicated.
Hope this helps!