You just fear losing him and are wanting more reassurance that he will stay.. hence the sudden and intense interest in what he's doing and what his emotional state is.
A small amount of this is to be expected in a relationship... however, when it becomes excessive it can smother the other person, make them feel that you do not trust them, and eventually lead to becoming a self-fulfilling prophecy (in that you fear they'll leave, then your insecurities disrupts the trust in the relationship, and thus they do leave - just as you had feared).
Regardless of what happens in the future... you have to relax and trust them now. Even if the relationship were to end at some point... let it end on something else - not on account of your insecurities. By trusting that your partner will be faithful to you, your relationship will have the greatest chance for success.
So basically, relax and trust that he won't leave or cheat on you. He's probably a good guy and not even thinking about doing any of those things. And more importantly - he's not the same as the guys in the past... this is a different relationship with a different man.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen