If I understand this rightly, it may be the wish for the ego to come in, the sense of "me", and also the idea of "mine" coming in. It seems like this person was able to balance it quite well and by reflecting of the good qualities of their partner, this could have been even more helpful to strengthen that sense of trust at that time. Due to our past conditioning, yes, the mind may reach out to our past habitual ways of reacting. That's where the power of practice is very helpful to balance this reaching out and tendency to fall into our past conditioning. It is the cycle of conditioning and the cycle of ignorance that is pulling you.
You all know about Mara. Mara will try to get you in any way it can. It's very tricky at times, so if we've actually practiced and even feel trust, Mara may see an opening, "Oh, this one! Maybe I get her in this particular instance, let's try!" So being able to withstand the tricks of Mara and past conditioning is very helpful to balance that tendency. Also taking joy within ourselves that we didn't fall into the old habitual responses can strengthen our new conditioning as well. And seeing more peace arising, helps us to understand how beneficial thinking in these new ways is.