Why? The poor dude misses his friends and family and it was already stated that he felt bad that his mom death was all for him and if that extra mile the mom went for him would go in vain he would feel horrible with a lot of guilt
Thats the thing, even if he goes back his mom will still be dead. And if he is alive( on earth or the titan healm) his mom won't have died in vain. So the only reason he "needed" to go back for is that he is missing his friends. But is it worth exchanging a life with the man that loves him ( and needs to marry someone from another healm and will probably die) just to see his friends??? It doesn't add up
AHHHHHH I'M SCREAMING
KOICHI IS SO STUPID IT MAKES ME WANT TO PUNCH HIMMMMM
POOR CAIUS AHHHHH(╯°Д °)╯╧╧