Leliana's crisis of faith is quite compelling stuff. I liked Leliana in Origins, and while it is depressing to see her so broken, it's great character development. The lady who would become Divine Justinia inspired her faith and restored her hope in a dark hour. To have her die in a way that seems pointless, that's a hard blow to handle, especially considering before this Leliana was one of the few who believed the Maker hadn't abandoned humanity.
Although I agree, I think was always a little bit like this before Justinia helped her, although to much a less extent. She was a bard for Majorlaine after all. Her time working in a Chantry, especially on the local level, has made a bit more positive. However, she has had several years working as a spymaster for Justinia now. This means the things she sees are much different. She isn't quite the person she was before when she was travelling with the Warden, and even then, that wasn't quite her either.
She's very much a person struggling to find who she is and what her place in the world is. I think Zevran pointed out out how alike they are, and it's more true than people give credit.