All sorts of things can affect a baby's mood. Or a combination of things. Teething certainly doesn't help, but he could also be hitting the stage where he wants to be more independent, but is also dealing with the onset of separation anxiety. He's still trying to figure out the world. Everything is a stage, you know. He's not going to still be crying every time you put him down or walk out of a room when he goes to college. Hang in there. It's tough, but it doesn't last. You'll miss it some day. ;)
Yes, there is always a stage, phase, etc. The come and go usually. I notice that if my son is being sensitive that there is something going on like teething, sickness, lack of sleep, etc.
I"m going through the same thing and my little one is only 5 months. SO challenging right now. She doesn't sleep a whole lot and is upset at the strangest times... like nothing will calm her down. So sorry you are going through this as well. I really hope it gets better for both of us.