Every person has different wants and desires. Some enjoy good food, others enjoy vacations, yet some others enjoy fine art. Although Olam Haba will be a spiritual enjoyment, how do we know that it will be something we will like?
true we have desires for food, vacations, etc. but that is not the innermost desire of our essence.
a human being is composed of layers of beings, from a gorilla (nefesh behemit) to a holy soul. these other desires are from outer parts of our being.
The Mekubal Rabbi Yaakov Adas writes regarding this innermost aspect of his being:
the most powerful, and most inner desire of a man's soul is to connect more to its source
source (see page 12). i.e. this is the desire of our innermost being, our innermost essence
and this is what olam haba is about as explained in the chovos halevavos shaar bitachon end of ch.4
Another reason is that the purpose of reward in Olam Haba is essentially clinging to G-d, and drawing near to His supernal light, as written "your righteousness will go before you, the glory of G-d will gather you in" (Isaiah 58:8)
I assume that the term, "reward" connotes enjoyment. We are rewarded for our mitzvoth, but we should expect to receive that reward in Olam Haba, not now. (Yes, many mitzvoth are rewarded in Olam Hazeh, but, Pirkei Avot, among other places, states that the expectation of the reward should not be the only reason to perform or discard a mitzvah.)
One source for this concept of receiving reward in Olam Haba is:
וְשָׁמַרְתָּ֨ אֶת־הַמִּצְוָ֜ה וְאֶת־הַֽחֻקִּ֣ים וְאֶת־הַמִּשְׁפָּטִ֗ים אֲשֶׁ֨ר אָנֹכִ֧י מְצַוְּךָ֛ הַיּ֖וֹם לַעֲשׂוֹתָֽם׃ (פ)
You shall keep the commandment, and the statutes, and the ordinances, which I command you today, to do them.
Rashi on Deuteronomy 7:11:1:
היום לעשותם. ולמחר לעולם הבא לטול שכרם:
And tomorrow, in Olam Haba, you will be able to take reward for them.
So, if you assume that rewards are pleasurable, then, yes, Olam haba would be enjoyable. As to whether you can refuse the reward in Olam Haba or whether it is "forced" upon you even if you don't want it ... Well, no one from Olam Haba has told me if that option is available ;-)