Inner motivation is often influenced by outside forces though.

Here's a good case in point:

In a class of 16 year olds I was asked to tell them how to become "successful". I asked them what they thought defined the word "success".

Of course we soon had a list of material things such as big house, flashy car etc etc etc, because this is what society tells us success really is, right?

I then asked them what their grandparents did and most said they were peasant farmers. So I asked if any of them were happy with their farms and did these kids consider their grandparents to be successful or not.

Of course there was a resounding YES!

As you can imagine, these kids were then thrown into total confusion because society tells them that success is quite the opposite to what their grandparents are/were.

The subject continued to be discussed for a further 3 hours, and I don't think I've ever had such a bunch of attentive kids in all my time teaching.

So yes, inner motivation is most important providing you are able to restrict outside influences.