' I Can't' is used by teenagers now-a-days to express their feelings towards particular things..it could be either unbearable or any feelings in that matter..such as opinions.
person 1 : "let me tell you a joke..what happened when the chicken crossed the road?"
person 2 : "What?"
person 1 : " The car hit it"
person 2 : "OMG!! ur sooooooo sad i can't"
two persons walking on the street pass a little boy in the age of 5 walking with his mother wearing a shirt which says.."Wuv me?"
the 1st person says to the 2nd person
1st person: " Did u see the shirt that boy was wearing?"
2nd person: " Yes.It was sooo cuuuuute. I can't!!!"