Why do exciting things "rock"?

Solution 1:

I think that rocks actually came about even earlier as a verb than the 1950s. According to the Oxford English Dictionary, by the 1920s rocks was a sexual euphemism in the US. In 1931, it started to mean:

To play or dance to popular music with a fast, vigorous rhythm and a strong beat, esp. exuberantly. Cf. to rock and roll at Phrases 1b.

Then, just a few years later, in 1938 a related meaning was added:

Of popular music: to possess a fast, vigorous rhythm with a strong beat; to exhibit the energy and drive characteristic of such music. Cf. rock n.3 2, rocking adj. 4a.

The earliest uses of the word in this sense are:

• 1938 Metronome July 21 Harry James' Lullaby in Rhythm really rocks.
• 1946 R. Blesh Shining Trumpets xiii. 309 The music‥jumps rather than rocks.

Rocks was first used to refer to jazz, and it was already being used to describe something with "the energy and drive characteristic of such movement". However, the OED writes that the slang usage emerged in the 1960s:

slang (orig. U.S.). To be full of energy, life, and excitement; to be excellent. Freq. in exclamatory phrase —— rocks!

The first known use in the form is:

1969 Times-Bull. (Van Wert, Ohio) 3 Oct. 2 (advt.) Bored? Uptight? In a box? Weekend bowling really rocks!

So the term originated with jazz, and took off from there.

Solution 2:

It comes from the sense of rock 'n' roll music. If it's good, it rocks.

rock
2 [ no obj. ] informal dance to or play rock music. • (of a place) have an atmosphere of excitement or much social activity: the new town really rocks | (as adj. rocking) : a rocking resort. [NOAD]

And from Etymonline:

Sense developed early 1950s to "play or dance to rock and roll music." Noun sense of "musical rhythm characterized by a strong beat" is from 1946, in blues slang.

If something has a strong beat, it is/was considered to be favorable or exciting.