The Baal Shem Tov once did the wonderful mitzvah of promising a couple who helped him in Istanbul, that they would be granted a first child, even though in Shamayim it was decreed that this couple would not bear any children. The Angels of Destruction complained about this and said, "by what right does he change Heavnly decrees?" Due to this complaint it was decided that his share in the Olam Haba (World to COme) would be taken away. The Tzadik was ecstatic at this result and said, "now I can truly serve Hashem lishmah, for no gain, since in any case I don't have a place in the Olam Haba anymore." (His share was returned to him, of course, in the end)
First of all, what a special middah of thinking only of Hashem and not one's self's needs. However, this is not a simple matter. I thought about this and realized that the tzadik may have said this more for our sakes than his own. How? Most of us, with our blazing yetzer haras (evil inclination) have an after-thought occasionally of, "what's in it for me?"
Notice that the Torah and the tzaddikim are always convincing us to keep the Torah and mitzvos, reminding us that the World to Come is the most wonderful place to be, full of the most elated pleasures and joy, unlike anything we have experienced in this world. Here, again, we are made to focus on the 'personal gain'. Yet, what is the real gain? Is it Physical?
The Moslem radical terrorists promise their She-ite followers that if they die killing a Jew or any other enemy, they are promised an after-life in a so called "Heaven" full of delights and virgin women. The animal side of them longs and lusts for that. How simplistic, superficial and downright stupid.
The question we Jews need to ask ourselves is, what exactly do we think the World to Come holds for us? I think that the Jews who are constantly wrapped up in gashmius - materialism, somehow think that in the World to Come, they will have a new Ferrari, a mansion, no need to lift a finger again and work, and the most wonderful foods and delights. I think that the Baal Shem Tov meant what he said for those who think there is a personal, physical gain in the World to Come. These such people really do need to work on doing mitzvos and Hashem's will for the sake of Hashem and not for any gain; Not for honor, or any future 'delights'.
Because if you think deeply on this subject as I have, the true merits of the World to Come are the kind of merits that really make you want to do mitzvos.
Let me explain:
In the Olam Haba, there is nothing materialistic at all. The pleasures are purely, 100% spiritual. They are the pleasures the souls will have in being closer to Hashem, holiness and purity than they ever were here in this world. Again, on a very simple level, reminding ourselves that Hashem is a synonym for "good", we are doing good in this world in order to be completely attached in every way, with every limb to "good".
To long for this is a very good reason for someone to do 'good' and mitzvos in this world. The gain is not physical. That person wants to do good because he reminds himself that this will bring him to such a tight bonding with the pure and beautiful good of the Olam Haba.
If you are yearning for these merits of the Olam Haba, please do keep reminding yourself of the 'gain' when you follow Hashem's will. If you're mixed up and think the World to Come is a physical gain of physical, ridiculous pleasures, you may need some more time to think this through. You don't really want to be like the She-ites...
To sum it up, one needs to be on such a high level of humbleness to be willing to give up his share in the Olam Haba for the sake of Hashem.
There are four levels here: The lowest is he who simply does not care / realize the beauty and truth in the World to Come so he sins without a care.
The second level is he who tries to do good so that he will merit 'physical' pleasures in the Olam Haba,
the third level - which most of us should strive for as it is wonderful: we should strive to do Hashem's will in order to merit being very, very close to Hashem in the Olam Haba
and the fourth level, is realizing that the best thing in the world to be is one who is on the third level, but we nullify ourselves so completely to the point that we don't care at all about anything we want - even if it is the right thing, we care only about living to serve Hashem.