Excellent suggestions for my future reading. There goes the budget (again).
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"I noticed my boyfriend shaving his dick and sac. Why do guys do this? - Amanda"
He shaves his cock for you. Yes, you read it right. He does that for your benefit. The old adage, "A picture is worth a thousand words" should help you understand what I mean.
I imagine that would be quite the irritant, no matter how much lube was used to mitigate it.
The smoother the shaft, the less irritation, the more enjoyment.

Guys shave their balls for their own benefit.
Hair acts as an insulation. It gets hot all scrunched up in a pair of briefs.
And then hot turns to sweaty. Sweaty balls are very uncomfortable. Scratching and tugging at them is kind of frowned upon (though it shouldn't be).
So shaving the ballsac reduces the need to do so.
It's cooler, less sweaty.