I´m not sure but i think we got to see the absolute best of both that year. Pace wise.
One is defending his pride, was the current world champion, on top of the world, notices a rookie matches him in everything.
He simply can´t let this young rookie come in and dominate him. It would be completely unacceptable because what does that say about his two recent titles? How valid are those? Was it the car or Alonso?
(just thinking objectively on how he might think, rather then my own opinion)
And for Hamilton, young guy, got absolutely nothing to loose, realizes he can match the 2 time world champion who reigned supreme for the last two years and ended Schumacher´s dominant years at Ferrari.
It must have been a massive pressure for Alonso really. I don´t think he expected to see that but instead thought, ok i´ve got a rookie, everything is fine, i´ll take this title and maybe he can play a support role.
Then in Australia, Hamilton goes around the outside of him like he´s been doing that stuff for years.
I think at that weekend Alonso realized it would not be easy this.
We know he´s not as comfortable with a strong teammate.