I suspect that Draco's feelings are in large part coloured by his intensely stressful sixth year, which at one point led him to break down in tears:

>"No one can help me," said Malfoy. His whole body was shaking. "I can't do it... I can't... It won't work... and unless I do it soon... he says he'll kill me...”

>And Harry realized, with a shock so huge it seemed to root him to the spot, that Malfoy was crying — actually crying — tears streaming down his pale face into the grimy basin.

><sup>*Half-Blood Prince* Chapter 24: "Sectumsempra"</sup>

Psychologically speaking, people tend to weigh [negative emotions more heavily than positive ones][1], and Draco had a *very* negative sixth year.

Another factor is that Draco constantly found his expectations disappointed:

 - According to his [Pottermore page][2], he was constantly being upstaged by Harry in things he expected to excel at:

 >Though he never sought fame, Harry was unquestionably the most talked-about and admired person at school, and this naturally jarred with a boy who had been brought up to believe that he occupied an almost royal position within the wizarding community. What was more, Harry was most talented at flying, the one skill at which Malfoy had been confident he would outshine all the other first-years.

 - Most of the things he supported or attempted ended up being disappointing at best, or actively thrown back in his face at worst

    - He, of course, often lost against Gryffindor (his most hated rivals) at Quidditch
    - The three people he hated most in the world were constantly being rewarded for breaking the rules
    - The Heir of Slytherin debacle in second-year, which Draco was *thrilled* about at the time, ended up being resolved with no harm done, except for a substantial blow to his father's reputation. To say nothing of what must have happened once Voldemort found out.
    - Also in second year, despite convincing his father to spend what was probably a tidy sum on seven top-of-the-line brooms, *he still lost*, which must have been a massive confidence drain; I'm sure Lucius wasn't pleased with him either
    - In fourth year he was *very* publicly humiliated by a professor, and got bailed out by the head of Gryffindor
    - Also in fourth year, despite Malfoy's smear campaign Harry ended up winning the Tri-Wizard Tournament
    - In fifth year he toadied up to Umbridge, which of course ended disatrously
    - Also in his fifth year, Ron ended up being the Quidditch hero despite Malfoy's "Weasley Is Our King" campaign
    - In sixth year, aside from the tremendous stress, he was basically ignored by Slughorn, a man known to respond to talent and success:

     >"Sir, I think you knew my grandfather, Abraxas Malfoy?" Harry looked up; Slughorn was just passing the Slytherin table.
   
     >"Yes," said Slughorn, without looking at Malfoy, "I was sorry to hear he had died, although of course it wasn't unexpected, dragon pox at his age..."

     >And he walked away. Harry bent back over his cauldron, smirking. He could tell that Malfoy had expected to be treated like Harry or Zabini; perhaps even hoped for some preferential treatment of the type he had learned to expect from Snape.

     ><sup>*Half-Blood Prince* Chapter 9: "The Half-Blood Prince"</sup>

     Quite a blow for a young narcissist.

 - Going over his Pottermore page, it's also clear that he was constantly envious of Harry for things that he (Draco, that is) had basically no control over; people generally like having a sense of agency, and feeling like his goals are unattainable, his best effort be damned, can't have been a terribly positive experience for the guy

A lot of these sound very petty, and they are; however, I can say from experience that [a hundred tiny annoyances build up over time][3], until optimism and enjoyment of your new school turns into resentment and misery.

  [1]:https://www.psychologytoday.com/articles/200306/our-brains-negative-bias
  [2]:https://www.pottermore.com/writing-by-jk-rowling/draco-malfoy
  [3]:https://en.wikipedia.org/wiki/Lingchi