A thing to remember about lead-acid batteries is that their life begins from when the acid is added, i.e. poured in, not when they're put into your car. This is why if you buy a battery in a supermarket it might not last as long simply because it's been sitting in the shop for a year or two.
I recall buying a battery from where it was manufactured - the dealer made them on site, it was a small factory - and that battery lasted in my car for seven years.
I've never had one last that long from anywhere else.
Another factor is the way a lead-acid battery is treated. Continually draining to almost depletion - as with hard starting on cold mornings - wears 'em out more quickly than when they're kept in nice warm environments and in fairly constant temperatures. Oh, and keeping 'em topped up with pure water (if they're not the sealed type) will ensure the plates are always fully covered with acid.