I have some misgivings about this answer but I think you are
If I hold you and you hold me, you might be alone.
"I hold you" refers to sitting in a wheelchair, "you hold me" indicates the individual holding the chair and operating it. If someone is operating their own wheelchair they might be alone (or there's a good chance they aren't) but the main point is to contrast the situations in lines 2 and 3 where they definitely are not alone.
If I hold you and you don't hold me, someone could be following you.
If the person is in the wheelchair but not operating it then somebody might be pushing them, essentially following behind.
If I don't hold you but you hold me, you're very helpful.
If somebody is not in a wheelchair but is operating one then they are probably pushing somebody else. In many circumstances, this is a helpful act.
If neither of us holds the other, you might have a reason to be happy.
If you are not in a wheelchair or operating one, you might be happier. This is the line I'm uncomfortable with - sure, it might indicate recovery from an illness but, more often than not I would say, individuals in a wheelchair are not unhappy.