It was all in the movie, young Spock has not mastered his emotions yet, young Spock loses his planet and his mother, old Spock tells young Spock to find the golden middle. Which part did you not understand?
Just guess how you would have imagined a younger Spock a year ago. Why did he break with his father and join Starfleet? Obviously because he wants to explore his human side, something he would never admit to anyone. Sounds familiar?