Euthanasia - animals and people.

Euthanasia/putting animals down.
Let me say before I start this post that I haven't decided what my opinion on euthanasia is. It's not something that I have put a lot of thought into. But I will post about is sometime soon (I hope).
Anyway so this thought occured to me in bible study this week that I'd thought I'd put out there. So when animals are really sick/dying we put them down to put them out of their missary (sorry to put that so coldly). They have no idea what's going on but we make the best decition for them, because we care about and love them and want what's best for them etc. So when it comes to dr assissted suicide when the person does know what is going on and is in full control of their circumstances why is that frowned upon? I know that there a lot of other contributing factors and obvioulsy there is a lot I don't know about this and need to look up. Just a thought I had that I wanted to share. Espeically implications there are as a Christian when forming my views about these things.