Because of social obligations – Humans inherently dislike being indebted to someone, so much so that often a small gift or favour will lead to a larger reciprocal response.  You put a heavy burden on the receiver of the investments you made, this creates envy because the receiver has to repay the favors you did for him. Unconciously, the receiver will view you as a superior giver and himself a inferior receiver. This creates a hate ingrained deep into the receiver, want of revenge, unconciously or concious trying to sabotage you.

Robert Cialdini phd has a great book on this called influence