Two: For the sheer romance
Maybe you're an incurable romantic who thinks that if one wedding is great then two must be fantastic. This approach seems like a recipe for disappointment. Surely a big part of the reason your first wedding was so special was precisely because it was your first wedding. It was (by definition) romantic. It was the real thing, the genuine article. Beware cheap (or, for that matter, expensive) imitations.
Three: For the attention
Some people are drama queens (or kings) and crave being the centre of attention. All the fuss and the prospect of acting like a princess (or prince) for a day can be irresistible to certain personality types. Renewing your vows might be just the excuse you need to throw a big bash.
Four: Having wedding envy
If you find yourself attending the second and even third or fourth weddings of friends and family members, you might become a tad jealous. It could well bring out your inner toddler: "Boo hoo. I've only had one wedding. No fair. Me want another one." But there's no longer a need to feel short changed in the ceremonial stakes. If you're unable to find yourself a fresh husband or wife, renewing your vows with the existing one is the next best cure for people with wedding envy.
Five: To assuage guilt
Sometimes one spouse suggests, agrees to or arranges a vow renewal ceremony as some kind of penance for misdeeds. I'm looking at you, David Beador. (He's the husband of Shannon Beador from the excellent reality television series The Real Housewives of Orange County.) All I could think when their surprise renewal ceremony was televised was that he had orchestrated it in order to make up for having had an affair. I felt really sorry for Shannon (my favourite housewife) even though she seemed so happy about it all. If my husband ever wanted to renew our marriage vows I'd be suspicious.
Six: To counteract marriage difficulties
It's no surprise to announce that sometimes the renewing-of-the-vows ceremony also does double-duty as a Band-Aid for marital difficulties. In such cases, it's as if the participants are protesting too much: "Everything's perfect here, don't you know. We love each other so much we're virtually getting married all over again." And, actually, it's just awkward. Because a renewing of the vows can represent many states but "perfect" is not often the one that first springs to mind.