Stretch marks come from getting bigger. I got them on my boobs when I hit puberty. I got them from gaining weight. I got them from being pregnant.
I figure... at least we have something to show for them - our sons! He was worth it! (And I have the lovely c-section scar to go along with the stretch marks). They fade over time. I think the only women who don't get stretch marks at some point in their lives are few and far between and just happen to be geneticaly blessed.
This is what I found online:"Stretch marks happen when the skin is pulled by rapid growth or stretching. Although the skin is usually fairly elastic, when it's overstretched, the normal production of collagen (the major protein that makes up the connective tissue in your skin) is disrupted. As a result, scars called stretch marks may form.
If you're noticing stretch marks on your body, you're not alone. Most girls and women have stretch marks, which tend to show up on the breasts, thighs, hips, and butt. Many women get them during pregnancy. And while they're more common in girls, guys can get stretch marks, too.
People who are obese often have stretch marks. Bodybuilders are prone to getting stretch marks because of the rapid body changes that bodybuilding can produce. Stretch marks also may occur if a person uses steroid-containing skin creams or ointments (such as hydrocortisone) for more than a few weeks, or has to take high doses of oral corticosteroids for months or longer."