Is it natural for a vampire to be alone? I don't think so. Almost every vampire in the Jossverse lives or travels with another. They live in pairs, families, clans, packs. Here are the glaring exceptions:
1) Russell Winters
As a MotW, Russell Winters doesn't get a great deal of characterization. We know he's sadistic, likes young women, and is the head of a major corporation. We don't see him associate with any other vampires. And he doesn't seem to know who Angelus, the Scourge of Europe, is--which either makes him young or isolated. But it could be easily argued that he has substituted a human hierarchy for a vampire one--especially considering his long-term emotional sadism with human victims.
2) Mr. Trick
Mr. Trick is a mini-Big Bad that never really clicked. We never see him emotionally resonate with anyone. He acts like a mafia lieutenant to the Mayor and perhaps gets some hierarchical satisfaction, but we never see it. When he arrives in Sunnydale, he doesn't arrive alone. He arrives with Kakistos. Oddly, when Kakistos is killed, Mr. Trick escapes with a female vampire. He could have just taken off without saying anything to her, just left her to her own devices or to be killed. But he takes her with him. If the cost is little enough, a vampire will save another vampire from danger.
Spike, out of all the vampires in the Jossverse, is the least likely candidate for solitude. And, so ironically, he is bereft of companionship. He tolerates Harmony (although he mistreats her) just so he won't be alone. He also interacts with the Scoobies more than he would if he really couldn't stand the lot of them. Why does he stay in Sunnydale after the Initiative is destroyed? He could scrounge anywhere for money and blood like Angel did during his 100 Years of Solitude. In fact, he'd have an easier time of finding an Initiative doctor in the DC area than in California. He doesn't leave because the only emotional ties he has are to Dru, Angel and the Scooby Gang. Yes, he may hate some of them--but hate is an emotion and it's better than nothing. At the mid-point of BtVS s5...well, we know the rest of that story.
Angel, more than any other vampire, has spent a great deal of time in solitude. But he did it because Darla cast him out. He also tried to reconnect with Darla two years later because he didn't want to be alone. She drove him out again. He spent the next 96 years in virtual solitude (he grudgingly did make connections during that time as can be seen in "Are You Now Or Have You Ever Been") fighting all his desires: the desire for blood and the desire for companionship. By the time Whistler found him, he was on the verge of psychosis in a New York back alley. He then agreed to the unthinkable--he reconnected with the world. Not with the demon world but the human one. And he did it on the side of the Slayer. But to connect is to open and to open is to feel pain...well, we know the rest of that story.
Now that Angel and Spike are together in L.A., how are they going to handle their reluctant companionship? Whether one says that that there's no love lost between them or that there is (and I'm an S/A shipper, so you know where I stand), there is certainly blood between them. And for vampires, that is an incredible bond. It could be one of hate, love, desire, repulsion (or all of them together). But it's there. They've both learned to substitute human companionship for demon. Will that be enough to break the bond between them?
I'm avoiding writing my open_on_sunday challenge. Better hop on that. Winter and/or steam? Ugh.