If you are nice to someone, or behave positively and pleasant towards them, there is a 50/50 chance that they will respond in a similar fashion.

If you act negatively, or are hostile or destructive towards someone, there is a higher likelihood that they will respond similarly.

Thus, it is always worth being constructive and positive, because it
a) encourages good behaviour in return
b) does not drag you into negativity, from where there is no real escape and
c) is more effective.

Also, it feels better.