Do you think Emma always loved Dexter and accepted being a friend??
I know she loved him like a friend forever, but did she love him throughout those twenty years too? What do you think?
I personally think she always wanted something more, but settled being his friend because she has loved him so much. I think that's why she dated Ian. It was an attempt at trying to move on from Dexter, though it didn't work. That's just my interpretation, especially because I feel the same way about my friend.
Dexter on the other hand took a longer time to realize that Emma was good for him. He just had to screw up his life to realize that.