Kermode's explanation is that it's to do with the human love of narrative, and our desire for that eponymous sense of an ending . Tick is a beginning, tock an end. "The fact that we call the second of the two related sounds 'tock' is evidence that we use fictions to enable the end to confer organization and form on the temporal structure." (The Sense of an Ending, p. 45)
(Indeed, if you wanted to get really clever, you could argue that 'tick' contains the idea of life - as in 'what makes you tick' or calling the heart a 'ticker' - and 'tock', with its echoes of both 'stock' - as in 'stock still' - and the German 'tod', that of death. That's my observation, not Kermode's, so don't blame him if you think I'm overdoing it.)
Like I said, it's a trivial thing, but it's a nice illustration of the way in which narrative does seem to be hard-wired into the species, a product of our awareness of mortality. I'm a relativist in most things, willing to accept that much of what we call 'human nature' is actually is a construct, specific to culture and circumstances, so it's always striking to come across something that appears to be genuinely essential.
This applies especially to artistic conventions, which arise for any number of reasons, some apparently universal, some very particular. I've heard a feminist comedian argue that jokes with punchlines are inherently male, aiming at a single explosion of laughter rather than a gradual build-up - I don't think I need to explain the analogy. The generally macho world of comedy (and stand-up in particular) has enabled a gender-specific pattern to pass itself off as a universal.
By contrast, some conventions do seem to be built in to our biology. The iambic verse line owes at least some of its power to the fact that it echoes the 'de-dum' of our heartbeat, rectangular paintings are satisfying because they correspond to the binocular field of vision. The three-act narrative structure, whether in Commedia dell'Arte, Noh Theatre or Hollywood cinema, appears to have a similar universality, although I'd argue that's more to do with mathematics than biology - stories need a situation, a change and a resolution, which makes three acts a convenient minimum.
'Tick-tock', of course, is a two-part narrative, going straight from birth to death, with just silence between. Kermode describes it as "a model of what we call a plot, an organization that humanizes time by giving it a form... Tick' is a humble genesis, 'tock' a feeble apocalypse." (Ibid, p.45) Your ear may be telling you 'tick-tick-tick', but the brain, with its love of story, has ways of making you tock.
