Dan Stevens: Charming in Downton Abbey

When Dan Stevens's character was introduced in the first season of Downton Abbey, I immediately took on to him since there were not many goodlooking men in the series and his character was less stuffy than the more aristocratic characters.  In season 2 of this favorite show of mine, we see him in a less sexy way because of an accident but his character faces a life-altering choice and I don't understand why the writers decided to inject a third wheel in what could've been an ideal marriage.