Rabbi Chaim Rapoport author of Judaism and Homosexuality: An Authentic Orthodox View was scholar in residence at my shul this past Shabbat. In a drasha he gave on Chumrot, he told the following story:
The Gemara in Brachot makes it clear that one must be very careful to pronounce the words of the Shema clearly and correctly, not slurring any of the words together or mispronouncing them. It lists several cases where the risk is particularly acute. One of these is in the 3rd paragraph of the Shema, where one must be carefult say tiZkaru rather than tiSkaru. When we pronounce the word correctly, we are saying we must remember the mitzvot. If we mispronounce it, it appears we are saying we must be rewarded for the mitzot. Pirke Avot makes it clear we should be like servants performing without expectation of reward, rather than those who only work to receive their reward.
Once when he was in a shul in Jersualem Rabbi Rapoport heard a man who was reciting the Shema with great concentration and attention. Sweat was literally pouring down his face as he continued. When he tried to say l'man tiZkaru, he wasn't happy with his pronunciation. 3 times he repeated the word, before finally saying in Yiddish "Lord, you know what I mean" and finishing the prayer.
Afterwards, Rabbi Rapoport walked over to him and asked him why he put so much effort into it. Surely there was no reason to work so hard at saying the prayer? The gentleman replied "But think of the schar (reward) I will get for doing this!".