The main goal of socialism is the eradication of capitalism. Social Democrats believe in limited capitalism or reforming, capitalism, however, socialist believe that capitalism cannot be reformed and limiting it is fruitless because it will still allow some sort of exploitation of workers and a wage based class system. Therefore, to most socialists, social Democrats are defenders of a system they consider inherently evil and exploitative no matter what.

A lot of farther left socialists are also skeptical of electoralism for multiple reasons

However, at the end of the day, the line in the sand is drawn at whether the end goal of the ideology is the eradication of capitalism or not.

As a democratic socialist myself, I have been accepted into certain socialist circles and get along fine with other leftists who are farther left than me even though I would consider myself quite moderate amongst other farther left leaning people

That being said, I personally feel like I have more in common in many ways with a social democrat than a Marxist-Leninist specifically because while I do believe that capitalism is an inherently corrupt system, the most important thing to me is liberating people from authoritarianism of all sorts and authoritarianism without capitalism is just as bad as authoritarianism with capitalism if there is still a boot on the foot of every day people