Holden Caulfield is a character who is defined by his isolation as much as anything else. The reason for this is that he has one by one ruled out relationships with every character because, in his opinion, they are phony. This however presents him with a massive paradox: on the one hand, he is desperate to reach out to somebody and start a relationship, but on the other hand he automatically expects that the relationships he forms will not end because, in his opinion, everyone is a "phony." Note, for example, the following quote in Chapter 9 when he tries to think of somebody to call up at the station:
The first thing I did when I got off at Penn Station, I went into this phone booth. I felt like giving somebody a buzz... but as soon as I was inside, I couldn't think of anybody to call up. My brother D.B. was in Hollywood. My kid sister Phoebe... was out. Then I thought of giving Jane Gallagher's mother a buzz... Then I thought of calling this girl ...Sally Hayes... I thought of calling... Carl Luce... So I ended up not calling anybody. I came out of the booth, after about twenty minutes or so.
Holden here is described as wanting to make contact with somebody as a result of his desperate loneliness, but he is unable to think of anybody he can actually call because his own issues have ruled out the friends that he has already made. It is slightly ironic that Holden, in one sense, could be accused of being the biggest phony of all, so hypocritical is he by his own standards. Holden Caulfield is a character who initiates temporary relationships because of the desperate desire he has to reach out and make contact with somebody else, but they are only temporary because by his own exacting standards any new person who enters his life is very swiftly labelled a "phony" and thus Holden leaves them straightaway and refuses to continue any relationship with them.