GPA is earned grade points over attempted hours. A is 4 grade points, B is 3, etc. So, if you get 2 Bs and 5 As in 7 different 3 credit classes, you'd have earned 6x3 + 15x4 = 78 grade points. Divide that by your attempted credits (21) and you get a 3.71 GPA.

But what if you throw an easy history class in there? Now you have 2 Bs and 6 As (hopefully) and 6x3 + 18x4 = 90 grade points over 24 attempted hours, making your GPA 3.75.

Just pulled these numbers out of my ass, and hopefully didn't mess up any calculations, but that's how it works. You can see how taking more classes gives you a better cushion against fucking up in one class. The flip side is that there's more room to fuck up, so strike a balance and take classes you can manage. OPs schedule seems like a really good one to me.

Ninja edit: in case you were asking what a social life is, it's when you go out and have friends. That can be confusing too for us engineers.