Instincts
The non-sociological theory is that of instinct. That is, the idea that we are somehow, as human beings, naturally programmed to behave in certain ways. This is an attractive theory because, if it is valid, it would explain why human beings generally behave in largely predictable ways.
From this point of view human beings are genetically programmed to do certain things in certain ways. So, for example, although a human infant is clearly physically helpless at birth, the child's parents, having been physically responsible for bringing him or her into the world, instinctively care for their child. You sometimes hear this particular idea expressed as a "mothering instinct" - the idea that women have an instinct to take care of their child and ensure that it is raised to a point where it can start to take care of itself.
The question of "instinct" is not, of course, as straightforward as the above suggests, and there are all manner or different arguments and interpretations surrounding the this basic concept. However, from a sociological viewpoint, there are a number of ways that the concept of instinct can be considered an invalid explanation for human behaviour.