Why do I get erections so easily?!

Hi LOL
It's just part of growing to manhood. Puberty is obviously well on the way for you. You will excperience this for many years to come. Have a look at http://www.puberty101.com. There is a lot of info here for guys at your stage of life.
All the best.
Norman

You neglected to mention your age - but if you are 21 or younger - then you are in puberty & your male hormones are raging from one end of the spectrum to the other, trying to figure out where to settle down. This can cause that little feller to develop a mind all it's own & there's nothing you can do but learn to live with it.

it means your dick is growing
but you must suffer the constant hard ons
jacking off will slow or completely stop the growth after awhile take
advantage of your sex hormones running wild and do masturbate

Perhaps the chaffing of the tight pants arouses you.

Dude thats normal. Guys get them pretty much the rest of your life.