When is the Right Time to Put an Animal Down?

We have a beagle/shepherd mix that we've had for what feels like forever. We got him from the pound when he was just a year old. That was twelve years ago. He was our baby before we had babies. He used to go everywhere with us before we had the kids. He's been through a lot with us. We've moved a few times and had a few kids and he's just been a really good dog. Other than being overweight he's been pretty healthy and he's always been a great dog. Our kids just adore him and we've never had any trouble with him ever being aggressive.

Well, here we are twelve years later and he can barely walk. We've had him to the vet numerous times and they say it's just arthritis. He gets a pill each and every morning and some days he does okay and other days it's just sad to watch him. Lately though he's had days where he can't get up at all. He's got one back leg that just doesn't want to work very well right now. When we try to help him up he gets a little snippy with us and that isn't fun for anyone. Yesterday was a bad day for him. He would eat his food and only stand on three legs. He had episodes where he was shaking and he laid around all day long. Late at night when my husband went to put him out for the last time before bed he couldn't get up so my husband helped him and he bit him. He bit him hard enough to draw blood and break the skin. This didn't make me very happy. It wasn't too bad, but what if it had been one of the kids?

So last night we decided that we'd call the vet today and just see what they thought and how much it was going to cost. I went to bed feeling bad because he's not miserable. He still eats and he does still get up to go outside and use the bathroom. It's very rare, but from time to time I'll catch him playing with one of the other dogs. It's things like that that cause me to wonder how you know when it's time. Not only that, today he's having a really good day. He's been able to get up okay and seems to be doing just fine. Now that could change at any minute and I know that.

I'm sure part of it is that I'm being selfish. We've had him for so long that it would be weird to no longer have him. Our five year old daughter absolutely loves him. She is constantly doting on him and I can't imagine what it will be like for her when the time comes.

So I'm asking all of you who have been here, how do you know when it's time?