A Tibetan mystic saying goes: We are here to realize the illusion of our separateness. The spiritual sentiment has a biological cognate. Our xenotropic drive — to merge with what is not us, temporarily in sex, or permanently in symbiosis or cross-species hybrids — is more than a metaphor. But it also offers spiritual solace. When we hook up with another, in sex or love (or, more rarely, both) we prove that our isolation is not permanent. In the fullness of time, we may all be linked. In the meantime, eros brings us together, making us more than we are alone. Cupid’s arrow, quivering into the heart of loneliness, kills us even as it sets us free.