Little kid hates bus and loves trolley

I had an interesting little encounter on the 21 bus this evening.  Two moms with their young sons, who were about the same age, got on the bus at 34th street or so.  The little Asian boy seemed happy - chatting with his mother (and not in English!), he clambered onto a seat.  The other boy, however, launched into nearly a tantrum.  "I don't wanna take the bus!" was the main phrase, interspersed was "I wanna take the trolley", and the occasional "I want to get off the bus!" peppered his stream of upset pleading for good measure.  The Asian boy, whom I assumed to be his friend, at first patted the seat space next to him, but the upset boy paid no heed to this offer.  The Asian boy's mother tried to assist too, making another paper airplane to give him, but this boy's mother simply decided he was a lost cause.  It was all rather funny to me since I much prefer being on a bus to being on a trolley in most cases and ways.  I wonder what could cause a kid that young to have such a strong preference for one type of Septa vehicle and against another?

I got off at 39th Street. :)