Please give some quotes to indicate that Holden is lonely in The Catcher in the Rye.
There are plenty of examples you could look at that show Holden is a character who finds it very hard to connect with people and is an isolated individual. Often, these can be quite subtle. Consider this example from the first chapter:
Anyway, it was the Saturday of the football game... I remember around three o'clock that afternoon I was standing way the hell up on top of Thomsen Hill... You could see the whole field from there, and you could see the two teams bashing each other all over the place... You could hear them all yelling.
Consider the significance of this image. What is important to note is that Holden is a figure who is by himself and not able to participate in the action of everybody else. He is shown to be aloof and alone and profoundly isolated.
You might also like to think of the way in Chapter Nine that he does try and reach out to people because of his loneliness, but that he is prevented from doing so because of his hesitation. Consider the following example:
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.
The way in which Holden's judgemental view of others combined with other factors prevents him from reaching out and calling somebody for help is a key contributing factor to his loneliness, although it is clear that he is in desperate need of somebody to speak to.