can think of three reasons.
1. Selfie is pretty much vain photography.
And weird faces are a way to GRAB ATTENTION.
2. The other reason might be, PEER PRESSURE. One person might say, 'Make a duck face' and when everyone follows it but one, it'll make them the odd person out. So, they jus follow the trend.
3. Pure FUN!
Well, that's why we make funny faces to babies and make them laugh!
Why not do the same with a selfie and make everyone smile? :D