My understanding from Romanov Autumn
, is that Alexander II liked to help women in "need". And by need I mean, women who seemed unhappy, in "need" of a rescue. Once he rescued Maria from her unhappy home, I believe after awhile he sought to "rescue" again. And well... along came Ekaterina.
Here is a quote from the chapter "The Tale of Two Women" - The wives of AII:
It was history repeating itself. In Darmstadt in 1839, Alexander has been captivated by a pretty fourteen-year-old who was obviosly unhappy and out of place. Now he felt the same awakening interest in another unhappy teenager.
As well, as the book notes, that Ekaterina looked much like the young Maria (to which I agree). Also, at this time, any physical relationship between AII and Maria was slim to none. Maybe more reason to find "love" again.