For most people, parties are difficult. Meeting just one person socially is difficult. It involves listening and talking to them, often without an agreed topic of conversation, while trying to maximise their opinion of you, yet seeming not to be concerned with any of these things. All should appear natural and easy. With close friends it is easy, of course, because you know more about them, which gives you many things to say. Nor do you need to worry so much about their opinion of you, which is already high.
This is why a pause in conversation feels so uncomfortable. At such moments, the fact that you are struggling for things to say to each other becomes too obvious to overlook. It loudly implies that you are not close friends or, sometimes, that your opinions of one another matter so much that it impedes your thinking, perhaps because you find each other attractive, or have something else on your minds.
To this mixture of difficulties a party adds the element of display. At a party you are required to perform the same series of difficult social tasks, but you must do them repeatedly and in public. In this way, parties make you aware of your place in society. Perhaps you are made aware of your failure, because you don’t know anyone, or because you find conversation difficult, or because you are very worried about what others think of you. You can fail by lacking the courage to attend, or by not being invited. On the other hand you may thrive, because you feel well known and liked. Parties publish your social performance for everybody including yourself to see, so the stakes are high.
Indeed it is the purpose of parties to make socialising difficult, in order to provide a test. A party is elite-level socialising, the competitive event for which the rest of your life is training. This truth gets hidden of course, because being good at parties means seeming not to find them difficult at all.