Eugene Onegin, Chapters 5-8

I've been awol the last few weeks, for a lot of little non-serious reasons, and I'm here to catch up.  So glad I haven't missed the entire second half of the readalong. 

This final half of the poem begins with a shocking event:  Eugene, in a fit of irritation with Lensky, dances and flirts with Olga (Lensky's fiancee).  Neither Eugene nor Olga sees this as anything serious, but Lensky does.  Quite uncharacteristically, it seems to me, he challenges Onegin to a duel.  Eugene haughtily agrees, and Lensky is killed.  Stricken with remorse, Eugene leaves his estate and spends several years traveling.  Shallow Olga recovers quickly from her loss and marries a soldier. Lensky is forgotten and his grave neglected. Tatiana, feeling that life no longer holds any joy for her, agrees to marry an elderly general, and enters St. Petersburg's social elite.  On his return to society, Eugene sees her at a ball, is struck by her grave beauty, and (at last) falls in love with her.  He writes her a letter that is a wonderful counterpart to the one she originally wrote him, in which he recognizes that he turned her away in the past only because of "my tedious taste for feeling free."  Though she still loves Eugene, Tatiana refuses to betray her husband, and Eugene is left standing thunderstruck.

Question from Marion, our readalong host:  Chapter 6 finds us in the middle of sudden disputes and high drama.  What might be the characters' motivations for such extreme actions?  Is it substance, or superficiality?  Is anybody right or wrong - and if so, who?

I want to respond to this because it's what struck me also--what on earth got into Lensky and Onegin?  I just cannot see sweet-natured, easygoing Lensky even noticing Onegin's flirtation with Olga, much less assuming on no evidence at all that a seduction was attempted, much less demanding a duel!  Likewise with Onegin--he's just too bored and can't-be-bothered-about-it-all to agree to anything as real and dangerous as a duel.  Not to mention his genuine love for his friend, the only sincere emotion he appeared to have up until that point.  I would expect him to laugh at Lensky's challenge, and tell him not to be ridiculous.  Honestly, I can't account for this part of the plot.