I'm not sure why you don't understand it.
Social justice causes people (especially straight white guys) to look inward and wonder to what degree their own privileges have helped them. This is pretty scary, because it suggests that their accomplishments weren't achieved by talent alone. A lot of people feel that mentions of privilege are attacks, and that these people must hate them for being white, straight, and/or a dude.
In addition, a lot of guys don't feel like they have any privilege. If you're a 22-year-old kissless virgin with few friends and a shitty apartment, and have never catcalled a woman, how dare those damn feminazis claim you have more power than they do?
The fact that some people described as SJWs do take an antagonistic or hateful approach doesn't really help things, but the main reason for bad feelings toward social justice are insecurity.