(a) Occasion: whether it is a birthday, wedding, picnic, college festival, etc.
(b) Society (people you may meet at the venue): friends, relatives, classmates, visitors, students from other colleges, etc.
(c) Availability: bought at a store, tailored, borrowed, etc.
(d) Fashion: designer clothes, casual, Indian formal, Western formal, etc.
(e) Your wish/whim: colour of my choice, style, etc.
(f) A suggestion or advice by someone (mother, sister, friend, etc.): Only advice by friends
(g) Any other than the above mentioned reasons: I would choose a dress that would suit me and set off my looks in the best possible way, even if it may be out of fashion. I would not go by whether it is expensive or branded.