Pomona (by my lights) is one of those lucky people who discover what they love to do and what they most value, early. It always shows, not just in their competence but in how they live life - usually quietly cheerful and unapologetically.
Snape, on the other hand, resented his circumstances and blamed them for his lot in life. His one attempt to better them by way of the Dark Arts and joining the DEs is akin to a Muggle get-rich-quick scheme; something that never works to the betterment of the person taking part. Sorrier and bitterly wiser after, he still saw nothing wrong in wanting to better his circumstances and resents being judged for it.
Pomona is not the sort to judge based on the past, but on what she sees before her. Hence, the basis of their pairing.
I'm glad you liked how their relationship developed and am gratified the story is one you wish to re-read. That is ever my aim, to write something people can come back to and find enjoyment (or distraction from their concerns) in it. Thank you!