You know what, I put that poorly. You're absolutely right, laziness is perfectly valid.

There's nevertheless a psychological difference between (variations on) "sorry dude, too many things on my plate this week" and "No.". There's nothing about the former anyone one can argue with, but it shows that you value basic human mutual understanding.

If the couch-owner truly lacks self-awareness, to the point where they constantly badger you with unreasonable requests (we all know that one guy), then a "No." isn't going to enlighten them either – in that case, you do them a favour by respectfully but explicitly informing them that their request is unreasonable.

Yes, but then laziness is not a highly valued human quality, so next time whenever you need help, people might feel about it the same way. And if someone gives that reason on a habitual basis, it might just actually mean he is not a helpful person. However, if you give them a short genuine reason of why you can't do it at that time, may be you'll have some hope.