Q:

Why does the average mind act to fulfill his desires?

Let's consider the following experiment: We tell a human to choose between two things, one is a "tasty" dish, and the other is a dish that doesn't is flavorless. Those two dishes have the same nutritional value. We also tell him that the first dish is unhealthy.
A priori these two things aren't very different, both are different forms of food that the human needs. However why will an average mind prefer taking the "tasty" dish, even do he was told that it is unhealthy? If he used his brain and thought about it for a moment he would have chosen the second dish, but he didn't, and privileged to fulfill his desire.
In the neurological sense, we can interpret this result as that the average mind prefers making acts that will let him produce some molecules such as the Oxytocin.

But why does the average mind fixes his life goal to act such that the "welfare" molecules such as Oxytocin be produced?

A:

Human beings operate on a foundation of instinct with an overlay of rationality.  We can, to a certain extent, override our instinctual preferences with rational decisions, but doing so in any particular situation isn't a given.  In general, our instincts act towards our benefit, but in a much less fine-grained manner than rationality, so there's always going to be gray areas where instinct pushes in one direction and rationality in the other.

