Often one is working when one goes to graduate school so, yes, one might have more trouble making friends. My undergraduate years I was living at school, too, so that meant there were other students living around me but with graduate school there often isn't housing there at school so you are on your own. Graduate school is more like "work", a bit more serious. You are studying what you work at/want to work at so there isn't the "general" education requirements where you might meet people who are studying something different.

In graduate school you pretty much have to make only A's and B's, you can't pass/graduate with C's or D's. However, usually the work is "automatically" deemed that level or they would not have let you in :-) so getting A's and B's is just doing the same level of work you did pretty much as an undergraduate. They do not explain things very much or do much "hand holding" in graduate school, you have to figure stuff out yourself. Some things you were expected to learn/know as an undergraduate you have to learn on your own, they don't have classes to catch you up, you can't "fake" things as well or "get by" as a graduate student :-) They might not have assignments, either, it's more like "read this book" and may only recommend texts rather than insist you buy books X, Y, and Z. Even with specific texts it is up to you to read/understand what parts you "need" to follow along in discussions, they don't assign pages 24-38, etc. and the "texts" are usually not textbooks, but "regular" academic books in the field and you have to figure out how they relate to the subject.

A social thing -- people are doing different things with the subject they're getting a graduate degree in and may be from different backgrounds. For example, most of the students in my graduate school were teachers getting an advanced degree because it was required of their school system or they wanted to get more money or better job. I was not a teacher, had never been a teacher or in a system where graduate school was expected. I was going to graduate school in that subject for my own purposes/I wanted "that" subject matter. So, the chances of "socializing" were more remote because of that, too, the backgrounds were different and there was no connection between our lives other than being in "this" course or "that" one together.