What if I have  testicular cancer?
Do you know for certain that you have cancer?
No I don't, but I have an ache in my testicles.
It's good to see a doctor if you have abnormal pain. Your doctor can diagnose you with more certainty and you'll be able to start getting the treatment you need if you're correct.
Nah, I don't like doctors. I'll just deal with it myself.
Do you feel confident your self-diagnosis will be better than the opinion of a someone who's studied the human body for for four years at a university and then studied medicine for four years at medical school? Think of it this way: how would you feel about having your uncle, a professional plumber, doing your taxes instead of a CPA?