This was probably hurt/comfort at its best. How does one continue to love a person when that person is no longer himself? You really feel the anguish of Zach, the conflicting and confusing emotions he goes through - it was quite heart breaking.
And Leith - despite losing his memories, the pull he feels toward Zach was beautiful. This was a great example of soul mates or destiny - the heart wants what the heart wants. So many feels and a great HEA.