Our Gemara on Amud Aleph shares Rav Yosef’s musings about his status as a blind person:
At first, I would say: If I hear one who says that the halacha is in accordance with the opinion of Rabbi Yehuda, who says: A blind person is exempt from the mitzvos, then I will host a festive day for the Sages. What is the reason? It is that I am not commanded and nevertheless I perform mitzvos. But now that I heard this statement of Rabbi Ḥanina, as Rabbi Ḥanina says: One who is commanded and performs a mitzvah is greater than one who is not commanded and performs it, I say: If I hear one who says to me that the halacha is not in accordance with the opinion of Rabbi Yehuda, then I will host a festive day for the Sages. What is the reason? It is that as I am commanded, I have more reward.
Sefer Daf al Daf quotes Eyn Eliyahu who asks on this from the teaching in Avos (1:3):
Do not be like servants who serve the master in the expectation of receiving a reward, but be like servants who serve the master without the expectation of receiving a reward.
If so, why should it matter to Rav Yosef whether or not he receives a reward?
Similarly, one might ask regarding the blessing in the Amidah for the righteous, where we ask that “Hashem give good reward to those that have true faith in him.” Why is this a point of interest to the righteous? Although, we might answer that here we are praying for them - they are not praying for themselves, so perhaps it is the right thing to pray that they receive reward, even if they serve God without expectation.
Regardless of the question about the Amidah, Eyn Eliyahu answers, that in fact Rav Yosef operated without any expectation of reward. It was merely out of humility that he framed his behavior as somewhat expectant of reward, because it is presumptuous to project such pure motivations. Ben Yehoyada answers that Rav Yosef was not referring to rewards in this world, or even the next. He was referring to the concept discussed in Avos (4:2):
בֶּן עַזַּאי אוֹמֵר, הֱוֵי רָץ לְמִצְוָה קַלָּה כְבַחֲמוּרָה, וּבוֹרֵחַ מִן הָעֲבֵרָה. שֶׁמִּצְוָה גּוֹרֶרֶת מִצְוָה, וַעֲבֵרָה גוֹרֶרֶת עֲבֵרָה. שֶׁשְּׂכַר מִצְוָה, מִצְוָה. וּשְׂכַר עֲבֵרָה, עֲבֵרָה:
Ben Azzai said: Be quick in performing a minor commandment as in the case of a major one, and flee from transgression; For one commandment leads to another commandment, and transgression leads to another transgression; For the reward for performing a commandment is another commandment and the reward for committing a transgression is a transgression.
In other words, Rav Yosef was happy to receive the reward of being inspired or receiving divine assistance to do more mitzvos, that comes from doing mitzvos. Psychologically speaking, this is known as Behavioral Momentum, which means once you are engaged in something you deem important or positive, it gives you strength and encouragement to do more and similar activities.
Despite the answers given, I believe there is a simpler answer. Rav Yosef indeed did not have concern for the reward in terms of receiving any benefit, but he did use the reward as a yardstick to determine the importance of the activity. He figured if there is more reward for the mitzvos that one does because he is obligated, that must mean they have greater impact. While the commentaries offer different reasons for why there is greater reward for a mitzvah that one is obligated to perform, once again, there is a basic logical reason. Imagine one person who is talking a vitamin because his doctor says he has a nutritional deficiency. Another person can take this vitamin too, but obviously, it will have less impact because he does not need it. So too, it is logical to assume that if a person is obligated in a mitzvah, that mitzvah is more impactful and beneficial to his soul. This is what Rav Yosef was celebrating.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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