Because everything is started from the small. If it is innocent or not, it is personal judgement. Take for example, biting nails. How small it may seem and yes it is innocent. However this can turn into a very bad and dangerous if you can not stop. My friend used to pull her one hair every time she is doing her homework (or during thinking) Now she has to wear cap for losing most of her hair.
It depends on what your talking about. Some people have addictive personalities and become easily addicted to almost anything. Some things are physically addictive such as dr.ugs, caffiene, ciggarettes and your body freaks out when you just stop using one of them. Other things are just habit forming...you get accustumed to doing things and it takes an effort to break your rituals and habits.
I would have to same the personalities is true. Its kinda how you are and how easyly you can get hooked on to things fast.
I would guess because they keep increasin and increasin and those small things are small things but on a higher level.
Perhaps they're not as small and innocent as you thought if they can turn into bad habits.