Why do I always think about him or her?

Something about him/her always brings you back to a time where you feel like you were more whole of a person, or had someone you can channel your attention towards, but understand that that person, even though they are gone, you are still there and still alive, and still have a plathura of things to find out about yourself and the world

Without more context the only reasonable conclusion that can be deduced from your question is the following. They are, for some reason, interesting enough for you to repeatedly think about them. Possibly as an idol, crush, celebrity, rival, mentor, etc.

because he's in your heart,you do feel him close to your heartyour brain and mind don't let you forget him,you love him

I think it can be about you hate, you love, or you have to finish something (like your problem and etc) with him/her :)

I think when we open ourselves up to someone we love there is a place reserved in our hearts. And in that place our minds recall those happy times and loving times in which we would like to always reside.

It is completely normal to think about someone after a break up or separation. It's how are emotions try to cope with the situation. You'll get through it amd eventually they will just be a passing thought.

If someone is constantly on your mind, there's only one explanation behind it. You care for that said person.

I guess it depends on in what way you are thinking about them. If you are thinking about a family member then you may just be worried about them, the same with a friend as well. But it could equally be someone you may have a crush on. I'd have a think about what it is you feel when you think about them and also what you think about when they come into your mind. Thinking about others is a great thing to do and it can show that you're a compassionate person especially if someone is going through a bad time themselves.

The reason you do is because that the person you look for comfort in the most. Which is okay however you don't want to have your world revolved around them. That's how you get hurt and no one want that.

You had a connection. Connections don't go away overnight, no matter how badly we want them to. However, since this didn't work out, think positively. Now you can find someone better.. "Don't cry because it's over, smile because it happened."