Here's a nice little video explaining scientifically (with fun little whiteboard drawings) why guys get morning wood aka nocturnal penile tumescence. See? Bet you didn't know the scientific name for morning wood. You're learning already. I didn't know it either. I think that name is so much better! Any time I can refer to my penis as tumescent, I'll take it!
We've all wondered why morning wood exists before. I know it's made for many confusing and awkward moments in my life. Thank you, ASAPScience, for explaining it to us.