How many answers are there to "Why?"

I think it's safe to say that these are the main theories about why Eric and Dylan went NBK:

1. One or both boys was/were bullied and/or humiliated and/or shunned to the point that his/their anger and/or frustration and/or alienation boiled over into homicidal rage.
2. One or both boys was/were mentally ill and/or defective/impaired* and/or suffered from a personality disorder (such as psychopathy).
3. One or both boys was/were unable to get laid, and was driven to kill by sexual frustration.
4. One or both boys was/were abused by family members.
5. One or both boys was/were raped by Officer Walsh.
6. The government brainwashed and/or manipulated the boys into going NBK to achieve some goal (gun control, growth of the surveillance state, and so on).
7. A "perfect storm" of one or more of these theories put together, and maybe some other factors, as well.

I always pick 7.

*The "mentally defective/impaired" category covers a lot of ground, including "The boys suffered brain damage after drinking water from a contaminated reservoir":
[You must be registered and logged in to see this link.]

Can you think of any others?

I would say 7.

Other factors including: both wanted to feel the thrill of shooting and killing, both wanted to feel powerful, and wanted to have power over those that they felt had wronged them, both angry about being virgins, Dylan angry about unrequited love and Eric angry about dates that were going nowhere, and Eric being angry at society and the way it worked. I would say those were the other main(ish) factors.