Why does Drummle almost come to blows with Startop at Jaggers' house in Great Expectations?

Bentley Drummle is a ponderous unpleasant character who is rude and sulky, seldom having anything good to say about anyone.  Startop is younger and generally more amiable than Drummle; both men, along with Pip, are students of Mr. Herbert Pocket.

One evening, Pip, Drummel, and Startop are invited to dinner at Mr. Jaggers' home.  During a conversation about the young men's rowing feats, Drummle becomes extremely agiatated, arrogantly asserting that "as to skill (in this area) he (is) more than (the others') master, and that as to strength he could scatter (them) like chaff".  Drummle is quickly "wound...up to a pitch little short of ferocity about this trifle, and he (falls) to baring and spanning his arm to show how muscular it (is)", with Pip and Startop feverishly and foolishly showing off their muscles as well.  Under the influence of a little "too much...drink", the boys' conversation then turns to the subject of money.  Drummle scornfully accuses the others of beign "too free with (theirs)", to which Pip counters by reminding him that he had borrowed money from Startop only a week or two earlier.  Drummle retorts bluntly that Startop would be paid back, and adds insolently that, Startop's generosity notwithstanding, he himself would never lower himself to lend even sixpence to anyone, and he dismisses them as "asses all".

At this point, Startop gently exhorts Drummle to be "a little more agreeable".  Drummle, resentful that Startop is such "a lively bright young fellow" in contrast to himself, responds "in a coarse lumpish way".  Startop then tactfully tries to change the subject, but Drummle, "without any threat or warning, pull(s) his hands out of his pockets, drop(s) his round shoulders, (swears), (takes) up a large glass, and would have flung it at his adversary's head, but for (their) entertainer's dexterously seizing it at the instant when it was raised for that purpose".  A fight having narrowly been avoided, Mr. Jaggers wisely announces that it is time to call their repast to an end (Chapter 26).