People change also because of others.
The guy/girl might be acting nice around you, but once you are not up to their standards (looks, personailty, morals etc), they act differently with you. Try to avoid you...etc.
People also change because in the beginning of the relationship, they try to put up a front like spoogenet said, and that is because int he beginnign is when the person tries to impress you. The longer you go on with the relationship, that person thinks that they can be their real self now, and that you'll forgive them no matter what since both of you are in "love" (which is a whole other story)
well you get what I mean (I hope)
