Because you are expected to live a semi-monkish existence even while acting as the galaxy's generals / police force / trouble shooters. Celibacy and a life centered around conflict and violence don't look to be a good combination.

Living too long as a Jedi will backward speak make you it will.

Because the Jedi can be pretty hypocritical about when and how their morality applies. Consider:

So having more than one padawan at a time is forbidden.

Falling in love and marrying is forbidden.

Trying to rescue your mom who may be in danger is forbidden.

Missing your mom when you are a little boy is frowned upon.

Just being afraid is frowned upon. (There is the whole path to the dark side thing though.)

Caring whether friends live or die is frowned upon.

Grieving when a loved one is taken from you is frowned upon.

And yet…

Trying to use the Force to compel someone to do business with you is acceptable.

Using the Force to rig a dice toss is acceptable.

Deliberately putting a child into a dangerous life threatening situation (pod race) to acquire funds, as opposed to using the Force on others to raise said funds, is acceptable. (Admittedly using the Force to basically mug people or win at craps isn't exactly moral but is it better to put a completely innocent child in danger?)

Leaving a child's mother in slavery when the cost of purchasing her freedom would have been negligible is acceptable.

Oh and a Jedi Master decided it was okay to execute a prisoner who, while clearly guilty, was apparently defenseless and at his mercy. So under the correct circumstances murder is acceptable.

Don't get me wrong; there is a lot to the Jedi philosophy deserving of respect. But for a group that takes such pride in its ethical behavior they do seem to pick and choose when to apply them.

Because a Jedi is expected not to feel fear, anger, hate, lust, sorrow, regret, or even love. What does that leave exactly? Mildly bored?

Because there is bound to be a prohibition against masturbating. I would explode.

Because the scariest bad ass in your entire organization is green and the size of a ten year old.

Because despite being granted occasional visions of the future not one of them even suspected their whole order was in danger of extermination.

Because you are not allowed to use your Jedi powers to pick up hot women in bars.

Because the only way to become more powerful than you ever imagined requires you to die first. Uh, doesn't that sort of defeat the purpose?

And the number one reason I would never want to be a Jedi…

After knowing Jar Jar for more than two minutes he was still alive.