I must commend you, Mr. Anonymous; Your candor in posing this question is rather awesome. You are asking me, in a public forum, specifically why YOUR penis itches. There's no beating around the bush, so to speak, with you. No "My friend has this issue." or, "What are some reasons A penis might itch?" You just come correct and direct: Funny Guy, save ME from MY itchy wang.

And a part of me feels bad, Mr. Anonymous, because I feel a question as pointed as yours should have an equally sharp anwer. Your penis itches because, you fell asleep naked on that poison ivy, remember? Your penis itches because it's currently wrapped in a scarf made out of that pink insulation stuff, remember? Your penis itches because you are allergic to tomatoes, yet you insisted on mixing a bowl of tomato soup with your penis, remember?

But, alas I don't have that one-to-one info about you. So I need to answer this more generally. Please know many of the reasons below may require further scrutiny, i.e. a professional assessment beyond my, or the good folks at Ask.com's expertise.

Off the top of my head, to yours. Your penis may itch from:
1. Having sex with a dirty vagina. I mean a dirty female with a dirty vagina (Herpes, and worse)

2. Having sex with a dirty man. I'll leave it at that. (Herpes, and worse)

3. Sweating after a workout and not showering for days (fungal)

4. Being a lazy post orgasmer and not cleaning yourself up

5. A mean allergy to new underwear

6. Sleeping on a dirty ass couch (pubic lice)

7. Eczema - reaction to new soap or pube shampoo

8. Scabies

9. Funky reaction to latex condom or some other penile wrap

10. Good old psychosomatics: you feel guilty having cheated on your girl, and there by illegally sharing your penis; now you are subconsciously creating this "itch" as a form of awesome self punishment.