Welcome! It's awesome to see someone willing to explore some kink for their loved one, often I see posts here about spouses who won't work to compromise -- so this makes me happy!

As for reasons, well, there are all sorts of things: first, find out what SHE wants to do. Is she on a diet, or working out? Breaches of those are good reasons to 'punish' -- it helps her towards her own goals and leads to fun too! Same with schoolwork or errands or whatever. You can also have spankings for random things: I forbid my wife from saying "no" (she can decline things, just not use that specific word). That's given me more opportunities than anything. Lastly, there doesn't always have to be a reason -- "I felt like spanking you til you begged me to stop" is valid!

Beyond this, just try to think of a couple routines to enforce: the no thing, a specific sort of greeting when she gets home (kissing your feet/crotch...asking permission to enter), or bed time ritual -- something that will reinforce "bdsm" on a daily basis. Then, try to think of random things to do every couple days. Call her up on her cell when she's at work or in a public place and tell her to immediately go to the restroom and remove her panties. Tell her to expect a spanking when she gets home, or masturbate while you're away. Things like that give the sense of domination without a whole lot of effort on either of your parts.

Hope it helps!