1. A lack of other things to do in a boring town.
2. A lack of funds, preventing actual activity (like films), but a wide and active social circle.
3. A broken or otherwise annoying home that must be escaped from.
4. School holidays devoid of actual vacationing/activity/homework, creating a sudden excess of free time.
5. Apathy; a lack of motivation, education, employment, a need for distraction in these circumstances.
6. Simply the desire to meet friends in a convenient place.
A mallrat isn't just a teen who hangs around in a mall for ages. A true mallrat displays some of the following characteristics:
1. You never actually buy anything, except perhaps drinks and snacks. You may not even bring money even if you could.
2. You treat the mall as an actual activity in itself, not just a way to meet others, use any facilities, etc.
3. You know the names of the security guards, where they walk, when the shops open, the menus of all the food/drink places, where the garbage is disposed of, the PIN code to the staff entrance doors, etc.
4. You get genuinely interested at the prospect of a new shop opening or a new stand being erected.
5. You beg or blag cigarettes, money, food etc from others.
6. You have 'your spot' where you and your friends always go. You get genuinely annoyed if you see others there and/or attempt to move them if you do.
7. Going to the mall is either so automatic that you don't plan it (you turn up like clockwork there at the same time every day and your friends don't even talk beforehand) or so important that you do indeed plan your visits constantly and build the rest of your life around it.
"Hey Moe, let's go and hang out at Fremlin's Walk."
"Again? You're such a mallrat."
Mallrats usually know everything that's going around in the mall (I,e: when the Easter bunny stand comes up) and knows about everything about the mall (what stores going where how long a store has been there and what store was there before that store)
also a kick ass movie by Kevin Smith