usually it is when you have quite fast weight gain or oss that these appear - like pregnancy and childbirth.
Best thing to do is get a really good moisturiser like Vaseline Intensive Care with vitamins and massage it in every day. Also using body gloves to help massage your skin when in the bath or shower - these not only get rid of the dead skin cells but also increase the blood flow and will encourage moisturiser to be absorbed. This can help with cellulite too.
Dont feel too het up about them ..... even the most famously beautiful women in the world get stretch marks (and cellulite too).
Hugs, xxxxxx