What is the biological reasoning behind infertility?

To potentially stop "bad" genes from continue?
To control the population?

What?

It happens everywhere in nature, and I was just wondering why, especially in humans, do we have infertility?

I know there are many WAYS to be infertile (LOVE King of the Hill!), but WHY are people (and animals) infertible? What biologically is the reason?

The term infertility is used in case of men.
The term sterility is used in case of women.
There can be scores of reasons for these two maladies.
In a way it prevents spreading of bad genes in the population.

As already pointed out, there are no clear biological evolutionary reason for infertility. Fertility and reproduction is a complex and intricate process. As such, there are many reasons for reproductive failure ranging from physical limitations to molecular and genetic defects. None of these things are a reflection upon the individual, just random effects.

Now, is infertility always a bad thing? Far from it. In some cases, it keeps people from bringing children into the world that they are not in any way equipped to raise and manage. In fact, under controlled conditions, deliberately inducing infertility into people before they reach maturity and keeping them infertile until they have matured sufficiently, meeting society's social/economic educational requirements necessary for maintaining a successful long-term relationship and family life. At the time that humanity deems a couple mature enough for marriage and family, the infertility could be reversed based upon some form of antidote. It would be a very effective means to: 1. systematically and ethically reduce the world population of human beings; 2. it would lead to the creation of better, stronger, marriages and family life; 3. it would significantly reduce the need for building more jails, prisons, and mental institutions, and, most importantly, it would save humanity from itself.

If that were true, then those people with really bad genes wouldn't be able to reproduce (like those who have kids that die within a year and whatnot). There are many reasons for infertility, but it has nothing to do with natural selection or anything like that. In my instance, I gained too much weight, and it made my cycles wonky and nonexistent. When I lose the weight, I'll likely have regular cycles again and be able to reproduce again.

It sounds like you are asking "Why do people (and animals) become infertile?" as opposed to "How do people (and animals) become infertile?"

If so, then there is no reason. It is the same as asking "Why do people catch measles?" or "Why do people go deaf?"
The answer is "It just sometimes happens that way"
There is no mechanism for population regulation involved.

Infertility occurs in an individual, not a population. Has nothing to do with genes or population control. Perhaps reading this will help you understand.

Infertility
http://www.mayoclinic.com/health/inferti...

Could be a narrow urethra like Hank Hill, or a tilted uterus like me! It could be any number of things.