If you think the reason is: We think a lot of shit about people and therefore speaking it all up would upset everybody in the end... What if there is one underlaying reason to explain both phenomena? Something like our terrible mental discipline? If all our thoughts were constructive, the best would be to speak all of them up!
What about the development of our brains? We have this genetic kit to ensemble a neural network so plastic it can adapt to almost any set of circumstances, and this information, by interacting with 'arbitrary' environments develop into functional operators. So we tend to relate these two instances (intrinsic and extrinsic factors) seeking for a directional cause going from one to the other. But what if we consider them both consequence to who you really are, or in another words, your decisions?
This is what karma means - Its what we have been doing acting as contrains for our current decisions ... Well, at least for me right now. But you'd have to assume reincarnation and a whole lot of other definitions of life and the universe to make this work. I'll deal with them later... =]