This card was given to you by an introvert. He gave it to you because he likes you, but is not feeling his best right now.
You might think that you should be trying to cheer him up, but since he is an introvert all of the friendly things you might try will only make him more tired and cranky. This is not because he does not like you. Really. In this state he doesn't want to talk to anybody. In fact the people he wants to avoid the most are the people he likes the most, since he doesn't want to be disagreeable toward them.
He is going to go into a cave now for a little while. This will make him happier and healthier. He recognizes that this sounds strange, but you're going to have to take his word for it. When he emerges, he will be very happy to see you.
Now go away.