Those in support of SYG are smart enough to imagine themselves or their loved ones in a situation in which they desperately need SYG.
Those who oppose SYG can only imagine other people using SYG to defend themselves, and they do not want them to be able to defend themselves. This implies that they either: want to oppress and control others, and/or want to protect those who want to oppress and control others, and/or are so terrified of unexpected loud noises that they are frightened to death from the very existence of guns in the world.
Do not forget ignorance or misinformation. Most of those who oppose SYG believe what they have been told by the MSM. They have been told that SYG means that people who get a concealed carry permit have a license to kill anyone they want at any time. That is what has been put forward, for the most part, by the MSM.
Most of these misinformed people would support SYG if they knew the facts.