Tenacity is normally thought of as a desirable quality. Tenacious people, it is thought, generally get what they want because they just keep trying. They don't get put off or discouraged no matter how many failures they encounter along the way.
Temerity, on the other hand is generally viewed as rash contemptuousness. It's essence is that of foolhardy contempt for the consequence of a decision or action. Notice the use of the word contempt. This is not the same thing as taking a well intentioned but uneducated swing and striking out. This is when you intentionally piss into the wind knowing full well you're going to get it right back in your face--and still you do it anyway.
