Hana and Kip

The reason Kip is leaving is that he's been given orders to go to Florence, and she can't stay there by herself now that her patient has died - she's a nurse & would probably be expected to rejoin the medical unit.

In one of their last, brief conversations, Hana says " I'll always go back to that church...to see the paintings". Kip replies, repeating part of what she said; "I always go back to that church". A little later, as he's leaving on his bike, Kip looks back at her with a faint smile.

I only really noticed those scenes on a second viewing of the film; it's very subtle but jumped out at me on the second view.

I've not read the book but think that brief exchange about the church leaves it open to interpretation while suggesting that there may yet be a future together for those 2 characters.

Another reason is Hana already lose couple of people she loved and close to her.
Kip is a bomb defusing expert where he risk his life everyday. So it's better to for them to be separated and live long and happy in a memory.