Here are some lines from Hawking and Mlodinow's 'new answers to the ultimate questions of life' book:
Philosophy is dead. Philosophy has not kept up with modern developments in science. Oh.
A law of nature is a rule that is based upon an observed regularity and provides predictions that go beyond the immediate situations upon which it is based. For example, we might notice that the sun has risen in the east every morning of our lives, and postulate the law, "The sun always rises in the east." This is a generalization that goes beyond our limited observations of the rising sun and makes testable predictions about the future. On the other hand, a statement such as, "The computers in this office are black" is not a law of nature because it relates only to the computers within the office and makes no predictios such as , "If my office purchases a new computer it will be black". [39-40]This is wrongheaded. Saying 'the sun rises in the east' has exactly the same truth-status as saying 'the computers in this office are black'; viz., a localised one. The sun does not rise in the east on Venus, after all. Just as the 'law' about the colour of the computers is falsified by going to an office with white computers; the 'law' about the sun rising in the east is falsified by going to Venus. Indeed, this Popperian notion of 'falsification' is weirdly absent from the book's discussion of what constitutes 'scientific law'. Really, not a single mention of Popper in the entire book.
One of [Aristotle's] predictions was that heavier objects should fall faster because their purpose is to fall. Nobody seemed to have thought that it was important to test this until Galileo. Lucretius and Democritus both disagreed with Aristotle about the nature of weight; Democritus probably and Lucretius certainly thought that unequal weights would fall with the same finite speed in a vacuum; and Simon Stevin showed that two objects of different weight fall down with exactly the same acceleration in 1586, long before Galileo.
The idea that the universe is expanding involves a bit of subtlety. For example we don't mean the universe is expanding in the manner that, say, one might expand one's house, by knocking out a wall and positioning a new bathroom. No shit, Sherlock.
Eddington visualised the universe as he surface of an expanding baloon, and all the galaxies as points on its surface ... if at some point two galaxies were 1 inch apart, an hour later they would be 2 inches apart. OK.
It is important to realize that the expansion of space does not affect the size of material objects held together by some kind of force. For example, if we circled a cluster of galaxies on the balloon, that circle would not expand as the balloon expanded. But you've just a few lines earlier said that the distance between galaxies—this circle—is expanding! There's a problem here. Hawking and Mlodinow want to insist upon this point, because 'we can detect expansion only if our measuring instruments have fixed sizes. If everything were free to expand, then we, our yardsticks, our laboratories, and so on would all expand proportionately and we would not notice any difference' . But the point of the balloon analogy is that spacetime itself is expanding (not, for instance, that the big bang happened in the middle of a cavernous empty space, filling it with matter; but that the big bang created the space as it expanded). If spacetime is expanding, then matter that is coordinated in spacetime would expand too. That gravity and atomic bonds set up a counterforce plays a part here, although there is nothing in the book at all on either dark matter or, more crucially, dark energy.
The boast at the beginning of this book is that it will explain 'not only how the universe behaves, but why'; and more specifically that it will answer the three-part 'ultimate question', viz:
Why is there something rather than nothing?But the answers Hawking and Mlodinow provide are weak. They dismiss theological answers on the (reasonable) grounds that answering 'how created the cosmos?' with 'God' only shuffles the question along to a new term ('so who created God?'). But then they do the same thing with their questions. For example, their answer to 'Why is there something rather than nothing?' is that though 'the total energy in the universe must always remain zero' it is possible to balance 'the positive energy of matter' and 'the negative energy of gravity': 'and so there is no restriction on the creation of whole universes' . But this doesn't explain why there is a balance of energy in the first place, or why the cosmos was pushed into this positive/negative state rather than the default nothingness. They answer their second question by invoking Conway's game of life, which shows that complex forms can emerge without a designer, but says nothing about the why. And they answer their third question by gesturing, rather vaguely, towards the anthropic principle. Bah.
Why do we exist?
Why this particular set of laws and not some other? 
To go back to:
Philosophy is dead. Philosophy has not kept up with modern developments in science. Dudes! You need to read more widely in contemporary philosophy.