Actually, it was because Descartes thought that animals lacked language and reason that he believed they were mere automata. (I say "mere", because we need to leave open the option, supported these days by, e.g., Daniel Dennett, that we are automata!)

As for "experiencing pain", we need to distinguish between actually feeling pain and (merely) exhibiting pain behavior. An automaton can do the latter; whether or not it can also experience pain is a separate question.

Consider a computer outfitted with a pressure-sensitive device connected to its operating system and an operating system that can be in one of 3 states. It begins in the "super-user-friendly" state and greets me with "Hi Bill; what can I do for you today?". I ask it to open my word processor so I can edit my philosophy essay on Descartes. It says, "Sure thing! Here ya go!". I edit for a while, and then hit its pressure-sensitive device very hard. This causes it to go into its "normal" state: When I exit my word-processor, the operating system merely gives me a new prompt. I log out. The next day, I log back in. This causes the operating system to go into its third state; it says "Rapaport. Oh yes; I remember you. You hit me. That hurt!".

Now, it seems to be exhibiting pain behavior. Does it feel pain? Is it anything more than an automaton? (Clearly, a lot more needs to be said, but this is enough to think about for now.)