Someone asked me, "What do you tell someone who asks you why you go out and help another Jew to put on tefillin?"
"There are many reasons. For instance, how many Jews are there in the World?"
"Because you answer like that you could have such a question. There is only one Jew in the entire world and each of us has a portion of that one body. The Jewish people are one. You help him, you help us."
There are other good reasons to help a Jew to do a mitzvah. For instance, what would you do if you saw a Jew's donkey fall down? For sure, you would run over and help him to pick it up. The Torah commands this. And if we are obligated to help another Jew to pick up his fallen physical burden, how much more so are we obligated to help him to pick up his fallen spiritual burden!
When you help a Jew to do a mitzvah that very mitzvah could be all that was needed to tip the scale of judgement in his favor and he will receive forgiveness or a great reward. You never know how much good comes from helping another Jew to do a spiritual deed.
You help him because you love him. He's your brother who doesn't know the value of doing the mitzvah. But maybe most of all, your reason for helping him is, he needs it. Is there any better reason than this?