Why do I always think about him or her?

You feel a connection with this person, or you want something to happen with you and this person, you might want to become close to this person

Getting over/ past someone can be hard. Whether it's a breakup or that person had left your life just know that there are so many other people that are there for you.

You think about it because somewhere you want yourself to be accustomed with the feeling that you had when he was here. It means that you are unable to accept the present. Once you have accepted it, it will go away. You can also try to divert yourself into hobbies so that your mind will be preoccupied.

because you have a system of desire: you want someone to empathize, sympathize with you, you want to be accepted, you want everything you've done get accepted, confirmed and affirmed. you want someone to communicate. you want to talk, to confess. and most of all, you want to satisfy your sexual need. if you don't want these, then you won't want that person,

Because they left mark that has made you think differently in good or bad sense but still makes you stronger minded.

This is difficult and not sure whether my reply will be an answer as such but more of an insight. You're thinking about them all the time and you just need to think about why? and the situation surrounding it. Do you have a crush on them? Are they single? Do they already like someone? Have they recently gotten out of a bad relationship? these are things that need to be considered before action. If you know them, form a friendship first, it's the best way in my opinion rather than jumping in.

Probably you love her or him. When we like someone, we always think about this person, always wondering how he/she is doing.

you are still there in the past and you use that relationship to avoid focusing on what you really want yes, you can recall good things, good scenes, good memories, what you loved that's investment but if you gravitate around negative or unfulfilling scenes or feelings, then you invest in your future frustration be very careful what you wish for i mean, where you look at (same thing :D) because if you decide to look at good things then it comes natural to avoid all bad things from that moment on and you must avoid looking at bad things to save time and attention for good things, past or future, real or imagined, doesnt matter it's vital for us to see the good, imagined or real investment in future positive

Well, clearly that person means/meant a lot to you. You keep thinking about them to register them completely in your brain because that gives you pleasure. Or if you're thinking about them after a bad experience, then you're trying to make sense of what happened...trying to figure out what you missed, or what went wrong on whose part.

Make beautiful, amazing, new plans... :-) they help you to put other things into your mind too! Thinking about people you want into your life is normal, but Albert Einstein used to say that we should tie our lives onto a goal, and not people