Endermen attack players on the same scoreboard team when the players look at the enderman.

Do this:
*/scoreboard teams add Friendly
*/scoreboard teams join Friendly @p
*/summon Enderman ~ ~ ~

*Look at the enderman (while in survival or adventure mode of course) and it will attack you. If you attack it instead of looking at it directly, it won't fight back.
*Summon in any other monster on the same team and you will notice they won't attack you as intended. For example a zombie: /summon Zombie ~ ~ ~