Human memory

How is it possible human memory to be so selective? How is it possible some memories to appear on the surface and affect us deeply and others we want to recollect to be insufficient to fight with the firstly described ones?

Is this fact a part of another fact - that our actions and feelings are pre-defined and no matter what we do, what is "written down" under one's name will definitely happen?

It takes huge efforts to overcome memories and associations flooding one's mind when hears about or sees something familiar.

I only wonder why I keep having bad associations for one thing or another. Does this have something to do with the fact that very often I react more emotionally rather than rationally.

So the question is: what is the correlation between memory and emotions. Is it 0 or 1? Or perhaps somewhere in the middle?