Wait, constant? There were only two example. The first occurred in the training grounds when Odin was lecturing Thor about focusing on what was right in front of him in Asgard. The second was when Sif called Thor out for being MIA all the time. Regardless, there's no love story there. Thor does not care for Sif in that way so how could they take it further? Nowhere in the film does Thor reciprocate any veiled affection.