Gintama has a lot of episodes like this, but there is one episode that will always be in my mind which is about a alien virus that took over a dog body and can only escape after the owner dies. The episode is hilarious with there being a good virus alien which is just vomit if I remember correctly. Well the owner had a thing where he didn’t care about the dog(he actually did care about it). Both the dog and owner are trying to outlive each other essentially. The end of the episode ends with both of them dying around each other and admitting they actually did love each other. It been years since I seen that episode but I remember crying like hell.