WHY does my baby cry all the time?

 He sees me, he cries. He doesn't see me, oh boy he cries. I put him down, he cries. I pick him up, he cries... I think you get the point. lol. I don't know what it is. He's 10 months in a couple of days. I thought he was teething, but nothing has come in, and he's been this way for a while. Whew! It's hard. He doesn't just cry. He can squeal and scream too. But when he's smiling like this he can do no wrong...