For car drivers, it is most likely twofold:
1/. Having lights on during clear daylight is often perceived by other drivers as an aggressive move. Whether this is your intention or not, you're reinforcing your presence and 'bigging yourself up', as it were, kinda like animals do to fend off predators. Drivers respond aggressively to this.
2/. So many drivers in the UK don't bother to ensure their lights are properly aligned, so even 'dipped beam' dazzles oncoming vehicles and at night can properly blind them. HID kits and the like just make it worse!!
Generally, drivers tend to under-estimate the speed of a vehicle approaching them with headlights on.
There's also the fact that it makes motorcycles harder to see and robs them of the visibility they create by having their lights on. To a driver pulling out from a side road, the bike's headlight will look something like this:
Yes, having DRLs may save the life of one car driver... but subtract from that the lives of three bikers... and often whoever SMIDSYs/pulls out on them. My bike weighs 42 stone - Unless your side window is rocket-proof, if you pull out on that bike you'll probably be as much a meat waggon write-off as I will!!
Read more here:
http://www.dadrl.org.uk/DRLstudies.html