Among my motorcycle history have been a W650, '07 Bonneville, and most recently a '73 R75/5. I just, plain and simple, love the style of bike from before the hyper-segmentation that we see today. These are what a motorcycle is supposed to look like to me.
Keep in mind that I'm 40, so not young anymore, but still too young to have had one of the originals when they were new, or to have even longed for them when I first discovered riding. I bought my first bike in 2000 long after the originals were gone, and as someone who came of age in the late 1980s, motorcycles didn't look much like these three.
Best year of riding ever for me was on the Bonneville. Took it everywhere, long trips, commuting, you name it. I know folks here complain that it's slow, heavy, poor handling, etc. For me it was perfect in those areas. I loved the handling -- it fit perfectly with my ability level and desires. I knew exactly what it could do and where its limits were, so I could ride it hard enough to have fun without the fear that I was gong to get over my head. If it had a larger gas tank (I want the range) I'd still have it.
I really liked the R75/5, and might get another. But the flip side is that these original bikes are now getting on 40 years old, and no matter how reliable some of them are, that's still a lot of years for a bike. I've got one old crock already (a '70 MGB) and dealing with the extra maintenance of an older bike isn't really appealing to me right now. My R75/5 really needed a full teardown, and I just couldn't do it, so out the door it went. Replaced by a F650 that I just haven't warmed to, so that bike will be leaving in the spring probably to be replaced by another Bonnie, or maybe a V7 Special, or maybe another /5 (i the new CB1100 had a larger tank, that'd be on the list as well). We'll see.
So I guess I don't see them so much as retro, rather as being what I want in a motorcycle -- just like I don't consider a modern Jeep Wrangler to be retro, it's just what a Jeep is supposed to look like.