The theories based on this specific scene is manifold and its open to interpretation, so please read further, for you have been informed that it’s purely my opinion.
With that out of the way, considering If you see meticulously, the moment when Cap is holding back Thanos, he is holding on to Thanos’s pointer finger and pinky. The pinky is where the soul stone is seated.
I believe that there is a chance that Thanos was seeing Captain America’s soul through the soul stone. I do not think that he intentionally tried to read his soul, but It happened.
In a blink of an eye, Thanos saw Cap’s whole life, from his scrawny days to his transformation and, most importantly, the day he sacrificed himself to save New York from Hydra’s bombs. If Thanos respects anything, it’s sacrifice.
I believe the looks that swept across the Titan’s face were that of awe at what Steve Rogers started as, intrigued as to who he came to be and then indifference as he continued his march towards his destiny.