This isn't exactly the most important thing I've ever written about, but then ... this is my blog, so I can write whatever I want.
Most manga series I read - or have read - contain side stories in all or at least some volumes. So does "Kizuna", one of the first yaoi-stories I've read (though not the first one). I prefer the two side stories in volume 6 and 9, dealing with a pair of hit-men. The story in volume 6 explains how the younger of the two - they both also feature in the main story of volume 5 and 6 - became a hit-man. The story in volume 9 deals with some mission they take over after they've gotten together again (during the main story of volume 6).
There are other side stories I like (almost) better than the main stories. One of the "Zetsuai"-volumes, for example, features the story of a bodyguard which is quite good.
And in addition they are much shorter, of course...