Reproduction

Nature has designed life, from the smallest microbe to the largest whale, to have one basic purpose. That purpose is to reproduce. Whatever else an organism does, reproduction is its basic goal. All else, from dolphins frolicking to the Pyramids, is gravy.
You may question this. So let's ask some questions:
"What do you mean by 'reproduction'?"
Basically, it's life making copies of itself. An organism is a combination of elements such as carbon, iron, hydrogen, potassium, sodium, etc.. How the elements combine into molecules, and how those molecules are arranged, determines what the organism is, whether a bacterium, bedbug or buffalo. The instructions for this arrangement are in the organism's genes. They provide the guidelines for everything from eye color (or even if there are eyes and if so how many) to the hard-wired instincts on how to react to stimuli. To make a copy of itself, the organism must create another organism that contains the same genes. This can be done asexually or sexually, but that's a topic for later in the chapter.
"But surely the basic purpose of life is to stay alive. Right?"
On the surface, it may appear so. Certainly most living things go out of their way to stay that way. They devote a lot of time and attention to self-preservation. However, what basic reason is there for staying alive? If nature just wanted life, everything could be immortal -- just do it once and leave it at that. But nothing is immortal; everything eventually wears out (or more accurately for life, dies). For life to continue, it must make fresh copies of itself. The purpose of self-preservation is to stay alive long enough to make those fresh copies. If an individual's death contributes to that individual's reproduction, it dies. For example, the male preying mantis is eaten by the female while inseminating her, providing her with nourishment she needs to produce and lay her eggs. However, in his sacrifice he passes on that part of his life that's important to nature -- his genes.
"What about all those other things organisms do? Eating, sleeping, building, travelling, etc.?"
Almost everything an organism does enhances its ability to stay alive, at least long enough to reproduce. An extreme example is the Pacific salmon, which devotes years to staying alive. It then engages in "kamikaze reproduction," (technically, semelparity) literally committing suicide in its efforts to reproduce. (Daly, 1978) All of its activities, from eating to dodging predators to travelling thousands of miles, aim at that final, fatal return to its original stream to spawn.
Other things that an organism does are attempts to increase its chances at reproducing. For example, among animals that reproduce sexually, males compete with each other to gain access to females. The competitions may be physical, such as butting heads, or psychological, by having the loudest roar. In any case, the competition is to get the status or prove the fitness that will entice the female to choose the winner.
"Well, that may apply to other animals, but how about us humans? We paint pictures, watch television, go jogging. What possible effect could these have on reproduction?"
An excellent question. The answer is very little. And what little effect there is subtle. Here we turn from the biological to the sociological basis of behavior. The more complex an organism, the more of its environment it can apprehend and try to manipulate to its own advantage. Often it does this through cooperation with other organisms. Humans, the ultimate in environmental manipulators, do more things, with more others, to more effect, than any other creatures on earth. You may watch television to relax; relaxation can benefit your health; the healthier you are the longer you may live; the longer you live the greater your chance to reproduce. Do humans think this way when they sit down to watch Gilligan's Island? Of course not. But the body might be saying, "Take a strain off," and, with many ways to relax, television may be the choice. Painting pictures may be relaxing, or provide money to buy food and shelter or prove fitness to a potential mate. Looking at the pictures may be soothing, or provide a more relaxing atmosphere, or be an investment. Jogging can improve health, and perhaps provide opportunities to meet healthy potential mates.
Humans are as much biological creatures as aphids or elephants. And, although humans have a conscious overlay of intelligence and culture, deep down in the subconscious, we have the same drive to reproduce our kind.