Occlusion. Severus is constantly occluding every single moment of the day – this is as much out of necessity as it is out of habit so ingrained it has become second nature. He keeps his surface thoughts aggressively inane – especially at Hogwarts, where he first developed the habit. 

The Marauders had such a preternatural ability to find him when he was alone as opposed to when he was with any of his dorm mates or friends ( which was exceptionally often – enough so that their ability to find him on his own was shockingly notorious and alarming ) he became convinced that one of them was a legilimens and began actively practicing shielding himself at school in the hopes of fending them off. While this ultimately failed, it only encouraged him to try harder – ultimately aiding in developing what would one day become one of the most formidable occlusion shields of the era. 

While this seems innocuous enough, it leads into particular complications when it comes to Severus when he needs to be open and willfully allow his mind to be penetrated – either by Dumbledore or Voldemort, the two most powerful legilimens of the age. 

While I have discussed the methodology of occlusion in depth already, it is paramount to understand that the tricks Severus employs on Voldemort are the very same ones he employs on Dumbledore, albeit for very different reasons. While for Voldemort Severus needs to present his hatred and his contempt, for Dumbledore Severus must present his commitment and his certainty and his absolute faith. 

Which frankly – he does not fucking possess toward Albus Dumbledore. He DOES NOT trust Albus, not with his life and certainly not with his emotions. Albus has never once showed him anything resembling respect or kindness – he is, to Severus, absolutely no different from Voldemort – he is powerful, and has the world convinced they must bow to his whims. If Albus is a better man – Severus has no reason to know it. 

So he focuses his energy on the people he does have that absolute faith in, the people he is absolutely committed to, and his absolute certainty that he will not fail them. He presents everything he is willing to present, and he conceals through redirection and the absolute and frank sureness that something is completely inconsequential in order to ensure it is overlooked. 

While after a decade of serving as a teacher Severus comes to respect Albus, by the time of the second war when he is consistently revealing himself to the man in order to ensure the war effort succeeds – he is also absolutely occluding and keeping Albus from anything that might force the two of them to acknowledge the grim truths of their positions. Severus considers it better to maintain the pretense that his life holds value to Albus Dumbledore than to know for certain that it simply cannot.