Tim Cooks speech is about finding purpose in life and how he did it, he wants to help and motivate the students by giving them his advices and experiences
to prepare them for their future. He wants the graduating students to know how important the mindset is, and that they need to believe in themselves throughout their life.
In the beginning of his speech he makes sure that everyone knows how happy and grateful he is to be there, and he also congratulates the students and tries to make them proud of themselves.
All of this is to make them feel equal to him, he wants them to see him as an equal and on the same level as they are.
This makes him feel more trustworthy and honest, about the stuff he says throughout the whole speech. He wants to help the students, he does this by talking about some of his own experiences.