i think because their life is always full of sorrow and beacuse it is so hot in the nether that there eyes water for no reason and that the ghast wouldnt be crying if it wasnt so hot in the nether but if they were in the overworld they would probably have a smile on their face and wouldnt have tears
they also are probably getting beaten by their parents and it makes them so sad that they cry
last of all i think they just read Where The Red Fern Grows

This is a really good vid nice work