I personally think there're two problems with the statement "I used to be quiet like you".
1. By saying this people imply (perhaps unintentionally) that being quiet is somehow a problem. More than that, the well wishers imply that they know best and that the 'quiet' person should change. It's arrogance really. Quietness is a problem only if it causes problems in life. And it's only the 'quiet' person who can decide whether and when their 'quietness' is causing them problems in life.
2. This statement also implies comparison. It compares quiet people (whose lives presumably suck) with the loud outgoing people (whose lives are presumably exciting and happy). Of course this comparison is drawn unintentionally, it's just that our society presumes that being loud, outdoing and competitive is required to succeed in life. Yet, comparisons are negative by nature. Also, we're all different. If everyone were loud and outgoing the world would be a terrible place.