We never fight, have a few minor spats, but nothing that really gets either one of us pissed off. I don't know if that's good or bad
That's not good. How often are these spats? What are they about?
She keeps an account, unknown to you, of every argument.
Every spat drops her interest level a bit. It depletes her "love bank", as it were. This is why women eventually divorce or end relationships more often then men, and when they do, the guys are usually taken by surprise when they're dumped. The usual story you'll hear is that they didn't see it coming, it seems to them that it was "all of a sudden". But it actually was a while in the making. They didn't realize she was only acting as if everything was okay. She wasn't nagging anymore (nagging's a MAJOR sign that it's heading south) She was acting as if fine because she gave up on the relationship some time ago.
The dumped guys will also say things like, "Our relationship was pretty good. Sure, we had the occasional argument, but just like any other couple." They are mistaken. If their relationship was good, she'd wouldn't have dumped him, so his perception is different than hers. He thought she thought the relationship was good too, but he was wrong.
The danger is not that she'll become complacent, but that one day, her love for you will have been eroded by the minor spats. So that's why I've addressed this concern, just to cover it.
A woman does not want to get bored. She needs to have emotional changes throughout her life to make her life interesting.
That's why they invented soap operas, chick flicks and romance novels, right?
But OP, it is true, never let a woman get bored with you. That doesn't mean you argue with her or have her on an emotional rollercoaster.
It's more about keeping her on her toes by being a challenge.
That means don't be predictable, or stay in a routine.
Recently heard a priest at a wedding talk about a church member who bought his wife a single rose on their first anniversary. two roses on their second anniversary. Three on the third, and so on, down through 50 years of marriage when he had 50 roses delivered on the 50th anniversary. The priest was using this as an example of the husband showing love and being sweet and romantic to his wife throughout the marriage...
... but I'm thinking that at some point, it's more likely the scenario was the wife's girl friend says to her, "So what are you and Harry doing for your 50th anniversary?" and the wife drones, "Well he'll probably just get me roses like he always does..."
Change things up. Surprise her. Take notes of what she likes, to surprise her with later on.
Date her.
Romance her once in a while.
Pursue your interests and passions foremost; stay on your path, your journey, your purpose in life.
Be masculine, so that she can be free to be feminine.
Respect her. No insults, name calling or slights.
Don't argue combatively. Seek win-wins, not merely compromises, because in compromises, one person always feels they lost.