Why do jeans get so loose after a couple of months?

it's because of the material. I have had this problem too, i would say to buy a stretch jean if you are looking for that tight look. I really like american eagle jeans, cause they last forever and they come in like every size imaginable. and try washing your old jeans in hot water, and drying them on a high setting, that should shrink them back up because they are cotton. though you might want to try on or the other first before doing both.