Why is it that sometimes whenever one sees a man or woman, interest comes up immediately for him or her?

I think that has to do with what you are looking at and what you see, perhaps even on a subconscious level. For one example, you may have had a crush on a young blond girl in the first grade and years later see a blond and your subconscious makes a connection with that memory without you ever being aware of it. It could be that the person is being friendly or have high esteem so that is what attracts you without you being aware of why that person attracts you more than someone else. there are many things which make up what we become in life and we are not aware of many of them on a conscious level unless we make an effort to do that. Take for example preferring one color more than another, one type of music apposed to another and so on. If we consider these things emotional triggers, the person you develop the interest or likeness immediately is pulling those positive triggers as opposed to negative ones. There are those though who believe that it is because you knew each other in a past life.:)