Much of the pain of loneliness felt by the unmarried—whether that of not being in a relationship, or that of being in a one-sided affair of the heart—comes, I think, from forgetting what it means to "be as little children."
Jesus told his disciples that "unless you turn and become like children, you will never enter the kingdom of heaven."
I think about that a lot, because children truly do experience a taste of heaven in a manner that can seem barred to single adults whose desires have "matured." They experience the most fulfilling happiness in the love of family and friends—without feeling the lack of spousal love that enters into adults in hope of marriage.
It is so easy to make a curious baby smile. When his eyes meet yours, all you have to do is break into an expression of joy, and the child's lips and eyes spontaneously melt into a look of delight. Poignantly, the same is true if you frown; his face will likewise fall, and he may even cry.
True, one could say that the baby's change of expression is not true empathy, because he does not yet fully understand you as an "other" outside himself. Likewise, were he old enough to comprehend your separateness, his choosing to empathize with you would add to him a kind of holiness that he could not attain without an act of will.
But even without the participation of the will, there is something God-like in the child's natural reaction that is all too often lost in the considered response of the grown-up. For God's response to our joy or sorrow is not a considered one. It is immediate. He rejoices with those who rejoice, and weeps with those who weep. Just thinking about our need for His redemptive sacrifice caused Jesus to sweat drops of blood, each one of which was enough to redeem the whole world. His goodness cannot help but diffuse itself. The baby's reflexive smile is an image of the smile of God.
The liturgy of the Mass forces us to "turn and become like children" when we say the Our Father before receiving Communion. We are reminded of our need for God to smile upon us—and are immediately rewarded as He bestows His peace upon us "in our day." That gift is, in turn, swiftly surpassed by the gift of peace in the world to come as well, through the Eucharist that brings heaven to earth.
For an unmarried adult, perhaps the most sorrowful words in the English language are the frighteningly popular expressions "only a friend" or "just friends." Only when we are adults do we add such qualifiers. As children, there is no greater joy than simply having a friend.
Although I have not heard the call to the consecrated life, I often think that priests and religious must be the happiest people on earth—and not just because polls suggest they are. Having chosen not to seek fulfillment in an earthly spouse, they are, paradoxically, able to experience shared joy and undiluted happiness with those close to them—much as children do with their playmates. Their relationships are based on a here-and-now appreciation of their friends, opening up the possibility of experiencing a kind of "untimed time" with them. It is fellowship lived in the present tense, freed from the limiting condition that the relationship progress into something more "meaningful."
There is, in fact, nothing more meaningful than being fully present for another in the same kind of chaste love that we will experience in heaven—when sex will be superseded by the communion it prefigures.