You can get full information from the link given below here is short intro
to your problem this could be a reason among various reasons
Youtube says:
Help Resources Home > Help Centre > The YouTube Community > Interacting with Other Users > Messages and Friend Invitations: How can I set up "friends-only" messages?
Messages and Friend Invitations: How can I set up "friends-only" messages?Individual videos
For individual videos, you can automatically approve comments from friends, but require approval for comments from anyone else.
To take advantage of this feature, follow the steps below:
Go to your list of uploaded videos (http://uk.youtube.com/my_videos).
Click the "Edit Video Info" button.
Scroll down to the "Allow Comments" section.
Select the radio button next to "Yes, with approval. Friends automatic:".
Click the "Update Video Info" button.
Profile page
You can also choose to allow friends only to comment on your Profile page.
To do so, follow these steps:
Go to your account page (http://uk.youtube.com/my_account).
Click the Channel Info link (http://uk.youtube.com/my_profile).
Next to "Who can comment," select the "Only friends can comment" radio button.
Click the "Update Channel" button.
General messages
To receive general messages only from your friends, follow the steps below:
Once you've logged in, click the "My Account" link in the upper right corner of any page.
Under Inbox, click the "General Messages" link.
On the left side of the page, tick the box that says, "Only allow my contacts to send me messages".
Click the "Submit" button.
Please note: If a user you don't wish to hear from is currently in your contact list, you'll need to remove them in order for this solution to be effective.
Also read this
Video Comments: How do I control commenting on my videos?Login to your YouTube account.
Go to the "My Videos" page (http://uk.youtube.com/my_videos).
Click the "Edit Video Info" button under the video you wish to edit.
Under "Sharing," you will find the "Allow Comments" options.
To require your approval for each comment, choose "Yes, with Approval."
To allow comments without approval, select "Yes, Automatic."
To turn comments off, select "No."
Make sure you click the Update Video Info button when you have finished.
You can also choose to turn comment voting on or off for each of your videos. You can do this when you upload, by clicking the "Sharing Options" window and choosing an option next to "Comment Voting." This option is also available after you've uploaded your video, by clicking the "Edit Video Information" button in the "My Videos" section.
Hope this will give you some idea.
nihon94@yahoo.com