"I've been struggling with my prayers." said the man to his Rabbi. "I find it difficult to motivate myself to pray properly. And sometimes, I just don't find myself in the mood to pray at all. What do you suggest, Rabbi?"
In other words, to do a mitzvah when it's enjoyable and inspiring is fine, but it can be difficult to know whether you're doing it for G‑d or to satisfy your own spiritual needs. But on days when you're not interested... and you do it anyways... think to yourself: "This is for YOU, G‑d!" And G‑d gets incredible satisfaction from such acts!
However, we should find ways to make prayer more meaningful for us.
We may not understand the meanings of the words we utter, but if our prayers are offered with simple faith and total sincerity they are beloved and acceptable.