a) He wants to show off his ballet- aerobatics- and parkour-trained back traps.
b) It's a balmy Chicago evening and and that's just what you do in the midwestern city famous for its ice cold temperatures for most of the year.
c) It will help the search helicopters buzzing outside to spot him.
d) He'd like to walk around his own house in his birthday suit dammit and wanted to test which floor would be out of range for paps with zoom lens cameras.
http://mumbaiboss.com/2013/08/14/aamir-khan-is-shirtless-in-the-dhoom-3-poster-because/