After a delivery, why do bowlers bother to stare at the batsman when they are on a loser?

Well, just studying your opponent(s) and not making underestimations is the best psychology. Try to work out their weaknesses and capabilities. It also depends upon the type of bowling. A spinner might try and kid a batsmen that he's going to bowl a googly, and then bowl one straight on. A medium pacer or a fast bowler may put a someone back in the deep to fool a batsmen into thinking a bouncer is coming, then slip in a slower ball. That way the batsmen has been beaten by reverse logic because he was expecting intimidation but undone by cunning if it results in a dismissal. Hence the psychology.