People are not like a chemical solution that turns a specific color when a specific chemical is mixed in. They are far more complex, and if you take 100 people and subject them to the same stimulus, they will all react in different ways. Some might react similarly and some may be dramatically different, but no two will be the same.
Nobody can know what your therapist felt or thought at that moment, so you are just collecting guesses people are making based on their own assumptions.
If you want to know, you should ask him. Maybe he was nervous, maybe he was dehydrated, maybe he was sleep deprived, maybe he was caught off guard, maybe he was excited, etc. All guesses.
One of the most important aspects of psychotherapy is being able to openly communicate your thoughts and feelings with your therapist. Regardless, of what they are thinking or feeling, you are doing yourself a disservice by not communicating openly with your therapist.
There are those writing that he can’t see you anymore. If that’s the case, he’s inadequately trained, and you’d do better to find someone who is able to handle all of your thoughts and feelings, not just the easy ones.
It’s rather common that patients love their clinicians. Aside from the fact that it’s often a very intimate relationship in which you likeky share things that you don’t share with anyone else, and there are obvious reasons to feel very close and even love your therapist, there is also transference. Transference refers to the experience of re-experiencing a past relationship or transferring those feelings into a present one (usually the therapeutic one). There are many kinds of transference as there are many kinds of past relationships. One that involves love and/or sexual attraction, is referred to as an erotic transference.
It’s very useful for the clinician to work with you around the transference, because it’s essentually an opportunity to work through past relationship conflicts in the here and now, and can be tremendously useful help individuals develop healthier relationships in their current life, instead of acting out pattens of dysfunction they might be carrying around with them from their past.
This is all to say your therapist should be able to take your confession of love to deepen the treatment and help you get more out of therapy. I encourage you to discuss your feelings and concerns about his reaction with him.