The reason it occurs is because the person being told to watch the vdeo is expecting something funny, which may not have been the case with the person who is telling the other person to watch the video (this person may have stumbled upon it, or if they created the video, overestimate how funny they really are)
Members of Youtube often destroy their chances of having their video go viral by saying things such as "omg go watch my video-it's so funny!" on the comments section of other viral videos-people are expecting something funny, and are let down when the video in question isn't as funny as they expected
Some videos are exempt from this effect, such as "HaHaHa" (the video of the laughing baby)
The YouTube Effect can apply to any other video, image, music, etc. site that shows related files or pages on whatever page you are viewing.