Daf Yomi - Yoma 72 - Shelo Lishma

The Gemara states that if one is meritorious through Torah study, the Torah will be a balm of life for him. If he is not meritorious, however, the Torah will be like poison. Rav Refoel HaKohen Hamburger writes in his sefer Daas Kedoshim that the term the Gemara uses for meritorious, zoche, refers to one who studies Torah leshmo, for its own sake. Rav Hamburger writes further, based on the words of the Rambam in Hilchos Talmud Torah, that the Gemara states that one should always study Torah shelo lishmah, because one who studies Torah shelo lishmah will eventually study Torah leshmo. This is true if ones intention is to ultimately study Torah leshmo. When one is young and has just begun to study Torah, he may require an ulterior motive to spur him on in his studies. Upon attaining the degree of studying Torah which is referred to as leshmo, the Torah that he studies is a balm of life. When one does not succeed in studying Torah leshmo, then his Torah study is akin to poison.

The Sefer HaChinuch in mitzvah fourteen, however, appears to maintain that even one who studies Torah shelo leshmo, without the intention of ever studying Torah leshmo, will be effected by his Torah study and he ultimately will attain the level of studying Torah leshmo. The Chinuch posits that this is analogous to one who collects taxes illegally on behalf of the king. Eventually, such a person will transform into a thief, as his actions will influence him negatively. Conversely, if one performs positive actions, albeit for the wrong reasons, he will ultimately be influenced to perform those actions altruistically.