Ikkaku's lived for hundreds of years wherein almost everything was the same--his reflection in the mirror, the face he woke up to every morning, the people he saw at work. Ikkaku's universe is...very small. And after those first few centuries, it gets sort of difficult to remember faces and the names assigned to them. Now, this problem hasn't really been an exceptionally large one, since Ikkaku spent a large chunk of his afterlife not staying in any one place for too long. Even so, though, even with that small universe that is, for the love of all that is good and holy, centered around Ayasegawa Yumichika, Ikkaku still learned to deal with people somehow. And he's actually not that bad at it. Ikkaku is, at a glance and little else, a charismatic man with a reasonable amount of stupidity. He can hold up a rational conversation--he's interested, but not too interested; he's got a decent sense of when not to ask questions and which questions he probably shouldn't ask. And in comparison to Yumichika, he seems pretty darn sociable, if not outright friendly.
So why doesn't he make friends?
Ikkaku has a very standard set of reactions when it comes to dealing with people. Someone like Yzak or Oscar freaks out at him, and he jokes around quite a bit but eventually, and in a way that seems sort of patronizing, tells that person to calm down. A cute girl talks to him and he takes any opportunity he can to compliment her, and he flirts sometimes if she's his type. And he treats retards like Yuusuke and Ichigo in essentially the same way, calling them punks and delivering necessary smackdowns.
What does this say about him? That he's on auto-pilot most of the time. He lives from moment to moment and relies largely on his instincts. Something in his subconscious yammers at him about cause and effect, what would happen if he does this to this person and so on and so forth, and he listens to that. Ikkaku knows what kind of person he comes across as: that charismatic and reasonably stupid man. And he makes a lot of his decisions based on what that person would do, otherwise he would walk away in the middle of conversations more often.
Because he's not interested in people. He doesn't develop personal investments in people. And it's not like anyone can tell when he's being earnest or not, because he's very rarely earnest.
Ikkaku has a tendency to lump people into groups because, firstly, these are, for the most part, humans. Ikkaku is not human. He has a very detached view of humanity overall--it's part of the job description. As a psychopomp back home, the living have been significant to him only when they...stop being the living, because that's the only time he has to deal with them. Other than that, people have never really been his problem.
Secondly, all positions of importance people may hold in Ikkaku's life have been basically filled. People just don't make impressions on him anymore. If he doesn't care to remember you, then he pretty much just doesn't remember to care.
Ikkaku isn't a nice person. He will never really claim to be a nice person. He will never want to be a nice person. To a certain extent, he does have a conscious influence over, say, whether or not he cares about someone. He's not naturally drawn to anyone in camp, and he has no real desire to be of any use to anyone. If someone he knows is in a crappy mood, and if he goes over and attempts to comfort that person, then he probably did so more or less on a whim rather than feeling any strong inclination to. Ikkaku believes everybody's got somebody in the end, which, sure, ties into his romantic feel-good side but also ties into the fact that he just doesn't see the point in giving somebody an ego boost that someone else who isn't him is perfectly capable of shelling out.
Ikkaku also really isn't very outgoing. In canon, he spends most of his time with Yumichika or by himself. He patrols Seireitei with Yumichika; he tries to walk off on his own when Renji asks him to be a captain; and when he's in part of the group designated to fight the Arrancar in Karakura, he meditates and thinks about how unsettling the atmosphere is while Hitsugaya bitches and Rangiku and Yumichika have a loud catfight a little ways away. Ikkaku by nature is actually quite rational. He's also shown himself to be genuinely sympathetic--but notably, I don't equate this emotional sympathy with emotional empathy, of which Ikkaku very clearly has none.
Yes, this would be the same person who posted in all capital letters about Himejima wanting his body. The fact of the matter is, at camp he doesn't take anyone or anything really seriously. He thinks of camp as a sort of extended vacation, and he combats its insanity with insanity of his own.