Sealed class

Question

am i write all class sealed if i not use base class

Answer 1

If you don't want your classes to be used as base class you mark them sealed, yes

Answer 2

why we all write default public or private instead of sealed if we don't use base class?

Answer 3

Publicity modifiers do nothing with sealing. You can have public sealed or private sealed classes.
And sealed modifier is not in default class template, so maybe that is why.