Snapedom

Several different lines of argument are used to explain why Severus is behaving in ways that Harry takes as hostile. One is that Severus had to act this way to maintain his cover as Death Eater: That when Voldemort returned Severus could point to his treatment of Harry as evidence that he remained a true DE and was never influenced by Dumbledore's agenda (and this would be supported by the testament of sons of DEs in his class if needed). A different argument is that Severus has strict and demanding standards (both academic and behavioral) as a teacher and Harry repeatedly fails those, thus bringing upon himself sarcasm, wrath, loss of points and detentions, as the case may be. And of course the argument more common among non-fans of Severus, that from the moment Severus saw the physical resemblance between Harry and his father Severus started taking on Harry his unreconciled enmity towards James (whether consciously or unconsciously).

This relationship becomes mutually hostile and results in Harry and his friends mistrusting Severus time and again - when they thought he was cursing Harry's broom and trying to steal the Philosophers' Stone, in the Shrieking Shack in POA, when they went to the Ministry in OOTP despite having already delivered him what should have been a sufficient warning to the Order and when Harry suspects Severus is a party to Draco's plot in HBP (well, he was in a way, but not how Harry expected). Severus' outburst in the Shrieking Shack ("... I have just saved your neck; you should be thanking me on bended knee!...") shows that Severus was offended by this state of affairs, he really expected to be trusted by Harry.

If Severus' behavior was strategic, intending to act the DE part - why would he expect Harry to trust him? Or is it that since their relationship already had a hostile start (whether because of Harry's conduct as a student or because of Severus' unsettled account with James) Severus decided to use it as part of his justification to Voldemort and the DEs (as we see him do with Bellatrix in HBP)? Did Severus expect Harry, perhaps with the aid of more trusted authority figures such as Minerva or Albus, to see the protection beyond the wrath and snark?

How does Albus fit in? In the early books he contributes to the distrust between the two - in PS he says Severus saved Harry so he could hate James' memory in peace, in POA he blames Severus for the need to make a daring rescue of Sirius. But later Albus reassures Harry that Severus was never suspected with any Dark activity since the first war (GOF), tells him how Severus warned the Order and searched for Harry and his friends in the forest, refrains from mentioning Severus' part in the matter of the prophecy until Harry learns of it himself (OOTP vs HBP) and tells Harry again and again that he trusts Professor Snape (HBP). Yet in OOTP he also says he expected Severus to have gotten over the past enough to teach Harry Occlumency successfully. So did Albus initially think he was supporting Severus' cover story but changed gears when he saw things were going badly? Or was he deliberately preventing the reconciliation of undesired hostility because it served his own plots?