It sounds like you and this person had more of a companionship and friendship rather than a romantic connection. While getting along and having things in common are important for a relationship to work, there also needs to be that romantic spark and passion. Without that, it's easy for a relationship to feel more like a friendship.
Some possibilities:
• You valued her as a friend but didn't have that romantic chemistry or attraction that typically characterizes a romantic relationship.
• The companionship and shared interests felt comfortable, but there was a lack of those butterfly feelings, excitement, and passion that come with romance.
• Over time, you realized the connection you had was more akin to a close friendship rather than a romantic bond.
• While you liked spending time together and got along well, there were not enough of the deeper emotional components that form the foundation for a successful relationship - intimacy, vulnerability, desire, etc.
Ultimately, if it felt like a close friendship more than a romantic partnership, then that's likely what it was - a close friendship. And there's nothing wrong with that. But for a romantic relationship to truly thrive there needs to be emotional, intellectual AND physical/sexual chemistry and passion.
Does any of that resonate with your experience in this situation? Let me know if you have any other questions.