Relationships yet again

Relationships are funny..
They are nice, good, happy and bang one moment and things change..
Things were going fine.. normal.. I was happy..
And then one sentence and everything changed..
I come down from a state of euphoria to the same ole shit hole i have tried so hard to overcome in the last few years.
The question here lies.. Why do we make ourselves do dependent on others that one sentence can make your whole world go topsy turvy?
But the bigger question is does the other person also feel the same..
having said that..am I overreacting to something that may not be so serious.. I dunno have too many questions in my mind..
A good conversation may help.. :)