Why after a year does my battery need to be replaced

There are various things. For most users 13 months would be early, but it could be possible if there's a lot of battery use. There are also random failures that can happen with lithium-ion batteries regardless of how they've been used. Is you're iPhone giving any particular message? Is the battery health showing a fairly high value (over 80%)?

sridinger56 wrote:

it says it needs to be replaced. Not excessive use. Just kinda miffed over it

Is it showing regular battery health?

It happens. I've had premature battery failure with an Apple device before. Apple doesn't really have access to any kind of special battery technology. Random failures can happen with any lithium-ion battery from any manufacturer. If if fails outside of the warranty (or AppleCare) period, then it's a reasonable cost to replace it.

Because batteries naturally lose their ability to hold a charge through use and chemical aging. It's normal.

Some people may get more life out of the battery based on their usage, some may get less.