It often annoys me when viewing YouTube videos that the quality of the video is low. I did a search and came upon this advice:

Speed

Your YouTube video quality may not even be a problem. Rather, the playback of your video is poor because of your slow internet connection. YouTube values speed over quality, so it will automatically present videos at a lower quality to eliminate buffering. At this point, you will have to manually change your video’s quality settings when viewing. Click the gear icon in the bottom right of the video, and select the highest quality. You will notice a longer buffer time, but the quality will improve significantly. If you uploaded your video at 1080p HD quality, then viewers with a mediocre internet connection can still access this quality. They will simply have to adjust the setting manually. However, some users will opt for YouTube’s automatic setting, that is, speed over quality.