I think also Leverkusen felt too confident and that is why they lost Europa League final. They had great season but they needed to remember that no team is perfect. Winning both Europa League and DFB Pokal would have been amazing but losing the Europa League final shows that anything can happen. Leverkusen did something great by beating team that had won 11 times in a row. One loss shouldn't hide how well they have done. I think they will try harder and maybe win the DFB Pokal which would still make this a great season.
Sometimes self-confidence is also an important factor in performing optimally because they are in good mental condition because they always win matches and have not been defeated, but it seems like their self-confidence is too high so they brag and feel they will be able to beat Atalanta easily even though no matter how confident they are should not underestimate their opponents and keep trying to perform as best as possible so that they can win this prestigious title, but it's useless because the Europa League match is over and Leverkusen have to accept their defeat for the first time in a very important match as well as an important lesson for Leverkusen not to demeaning his co-stars even though they've had a great season.
Hopefully in the DFB Pokal Leverkusen can win the title, their opponent is not a big club, Leverkusen should be able to conquer it, but they still shouldn't underestimate their opponent because surprises can still happen like the previous match, but what is clear is that Leverkusen will be more alert and will not be careless like their match in the Europa League previously, even though he failed to win the Treble Winners, at least Xabi Alonso had to optimize to win the Double Winners.