Stretch marks are a type of scars that are formed when our skin stretches or shrinks quickly. This kind of abrupt change causes the collagen and elastin, (that support our skin) to rupture. As the skin starts to heal, stretch marks might appear.
Also, not everyone develops these narrow bands on their skin.
If one develops stretch marks, then they are most likely to get them during:
Growth spurts because of puberty
Rapid weight loss or gain
Weight training during rapid muscle growth