I Love This.
It has taken me seeing this a few time to fully understand it.
It's just saying that true friends don't leech off of you. True friends are there when you need them the most. Plain and simple. Apparently some people just assume it is meant to be creepy, and has no actual point, but it does.
The man believes the tumor will be his friend, but it just leeches off of him. This is similar to a friend in our world, who doesn't really care about us, but just wants to take advantage of what we have.
In the end, the dog shows itself to be the man's true friend. This is made apparent by the dog sacrificing itself for the man. True friends in actuality help their friend's in their time of need, just as the dog did here for the man.
I love this cartoon, and find it a very exotic portrayal of such a simple thing. Spot on, well done!