Outside the pit of shadows he wrested himself from, Ulquiorra spent all of his time as a hollow alone. As he was a most unusual hollow with no mouth, no ears, no nose, no other way to interact with the world around him than by sight or claw, he had no real desire to seek companionship in the first place. There was nothing he could do to make them understand him, or for him to understand them.
Most other hollows kept well away from him upon sensing that heavy reiatsu anyway, so thick it was with despair. None would ever come close. And if they tried, the sheer density of that despair would make quick work of them – in one sense or another.
Even upon first becoming an Arrancar, he was alone. He holed himself up in his prison of crystal, content to stay in that isolation until the end of days, hoping to lose himself in its cold embrace, to finally wither away into the nothingness he so desperately craved.
Rumor spread that the giant mass of crystal branches he attempted to die in was a cursed place, unsafe for any hollow to approach no matter how brazen.
So they all continued to stay well away.
