There is no rule in the laws of physics that says that human life is valuable.

However, when people hear this, there is a flinch response to this: 'human life is not really valuable, we only think it is.'

That is wrong. Or, at least, not justified. In fact, the laws of physics and the universe at large are not qualified to comment on moral issues. Value, as a universal value, is not well defined. It is subjective, although not totally -- it can be argued about between people, and minds can be changed. Morality is something that exists only when intelligent agents are concerned. I value other humans because I value myself and logic dictates that they are not so different than me. As for why I value myself... well, that one may be down to basic architecture of the mind.