Colon and underscores

[Igor]

It would be really nice to know the reasons behind some syntax
features.

What is the real purpose of the colon ":" after each if, def, etc...
Is it really required and cannot be made optional?

Why__ do __Python__ programs __need to __look__ like __swarms__ of
__underscores ? Is it OK that one must dig in underscores instead of
having a normal English word for private symbols, for example?

Regards.