The tzadik (the righteous one, the ideal spiritual level of every Jewish soul), in his unconditional devotion to serve and become one with G-d here and now, possesses no intention of receiving the reward of the World to Come (man's own [as a separate entity, as it were, from G-d] ultimate good).
Rabbi Shneur Zalman of Liadi used to say, "I desire not Your higher Garden of Eden; I desire not Your lower Garden of Eden; I desire only You and You alone" [See HaYom Yom, 19 Kislev].
In one of his Chassidic discourses [Ma'amarei Admor HaZaken, HaKetzarim, p. 461], he explains, seemingly paradoxically, that meriting children (both physical and spiritual) depends upon one's unconditional devotion to, with no other intention but becoming one with, one's spouse (on the spiritual plane--Jew to G-d), thus implying that children are in fact the true and ultimate realization and embodiment of marital love and devotion:
A way to merit children, and the meaning of bearing a son and daughter.
"Son" and "daughter" refer to [the two primary emotions of] love and fear [of G-d].
The story is told in the Midrash of a Jewish woman who was married for many years to her husband, but had not had children. He decided to divorce her, so he went to Rabbi Shimeon bar Yochai, of blessed memory, and the rabbi told him that just as they celebrated with joy their mutual bond when they got married, so should the severance of their mutual bond be celebrated in joy. The husband therefore prepared a great feast, at the height of which he called his wife and asked her in his joy to chose whatever she desired of his possessions to be hers, and said that he would not refuse her anything.
What did she do? She served him so much wine that he got drunk and fell asleep on his bed. She then told her servant to take him on his bed into her bedroom.
The following morning, when he awoke and found himself in his wife's home, he asked her why he was brought there--wasn't it clear that he intended to divorce her? She replied, "Didn't you tell me that I could take whatever I wanted? I desire not gold, nor silver, nor precious gems, nor pearls. All I want is you. You yourself are the sole object of my desire."
When the husband heard this, he became once again enamoured of his wife, and took her back as before. And in this merit, the Holy One, blessed be He, granted them children.
So it is with regard to [the service of G-d]. As it is stated [in the Song of Songs 8:2]: "I will serve you aromatic wine, the fragrance of my pomegranates"--this refers to the fact that the least worthy of Israel is full of mitzvot as a pomegranate is of seeds. [The bride, Israel, serving the groom, G-d, means arousing Him] to descend [and dwell] amongst us, for [as it is said in Psalms 73:25,] "Who have I in heaven, and I desire nothing in addition to You on earth." Meaning: I have no desire for any good thing or delight, neither in the higher Garden of Eden, nor in the lower Garden of Eden, I desire nothing but You alone.
In this merit, one will "bear" spiritual "progeny"--a son and a daughter, i.e., love and fear, as explained above. And also on the physical plane, "he will see [i.e., have] offspring and [merit] long life" [Isaiah 53:10].