I think the most trying times for batteries are the winters (assuming the usual very cold, midwestern winters up along the northern tier) -- these really kill batteries, albeit it slowly even if their actual time of death occurs later, the following spring or summer. If you've kept your car outdoors overnight in the winter, yours may be on its last legs.

As an aside, I put a piece of electronic hardware called a desulfator on all my cars' (and garden tractor's and boat's) batteries. It's not cheap, but I believe it really extends their lifespan.