Liverpool were looking to close to within four points of Premier League leaders Chelsea at Anfield, but found themselves 2-0 down after Fernando Llorente's quickfire second-half brace.
Roberto Firmino's fine double brought the hosts level and it appeared as though Liverpool had the momentum to earn a comeback win.
But Liverpool were caught out by Gylfi Sigurdsson's sucker punch on the break in the 74th minute to suffer a first home league defeat since January 2016, and manager Klopp was left frustrated by his side's wayward concentration.
"In the first half there was only four or five moments. We expected it because we had to stay patient and create the moments where we could have scored and we couldn't do it," Klopp said.
"We needed what we didn't have in the first half, the kind of last punch in the right situations. We lose one challenge then Llorente is in front of the goalie. The first ball, then the second ball comes back. It's too easy, obviously.
"[For the] second goal, we can defend the cross, it's why you should avoid crosses.
"These things happened. We came back and played really good, we deserved the first goal, deserved the second goal, and the most disappointing moment was [Swansea's] third goal because I have no explanation at this moment, there were four or five moments to have a challenge and one guy [Sigurdsson] is free.
"They had luck in the right moments, they weren't in front of our goal maybe more than the three times [they scored]. Is it fair? Maybe not. But is it deserved? Yes, because they took their chances."
Klopp also insists there was no hint of complacency from Liverpool against a team that travelled to Merseyside bottom of the Premier League.
"No, if you watch the game and I hope you did, you cannot ask this question," he added.
"The situations with the goals was disappointing, [but] we didn't think about anything else in this game."