Unfortunately, it is commonplace for mathematicians who are at the peak of their game to lose some sight or perspective of their time as a student and thus appear as slightly arrogant - even then, they were probably excellent students and so never had your perspective in the first place. Sometimes they are explicitly arrogant (rarely though in my experience) but other times they are more subtly arrogant, perhaps unknowingly. For example, you may be in a lecture and someone might ask a 'trivial' question to which the lecturer may respond by explicitly saying that the result is trivial. The consequences of this are larger for the student than for the mere lecturer whose mind is probably more focused on their research duty. Indeed, the student has just been told that the thing they didn't understand was basically easy in the first place. The resultant feeling can be, understandably, one of demoralisation.
Fortunately, most mathematicians - from my experience - are very willing to help you and try to be as kind as they can in the process. Just remember that with Stackexchange most of them are academics, some of which work at world leading institutions. A degree of oversight with regards to your feelings can come from this, especially in such an analytical subject as mathematics.