I suggest you to download the videos you like watch in HD version and then view without distraction there are many youtube videos downloader that you can use for this purpose. here you can try few best downloaders. http://feedsyouneed.com/youtube-music-downloader/

Hi Kriti,

To give you the best viewing experience possible on your computer, YouTube adjusts the quality of your video stream from standard definition (such as 240p or 360p) to high definition (720p or 1080p), based on the speed of your Internet connection (bandwidth).

You might not be able to play the video in 720pixels because of the below reasons.

• Video player size: By default, the quality setting will be on “Auto” and YouTube will use the highest quality based on your video player size.
• Quality of the original uploaded video: For example, if the video was recorded in standard definition, it will not be available in high definition.

You can try the below fixes to resolve this issue and enjoy watching the HD streaming of videos.

Fix 1: Change video quality
Change the video quality by going to the gear icon in the corner of the video player, and selecting your preferred quality.
On mobile devices, adjust quality settings by touching the menu icon, then the gear.
This option isn’t available on all devices (e.g. TV/game consoles), or connections (e.g. iOS on a cellular network).
If you’re signed in to YouTube on your computer, you can set default video quality preferences for all videos. 

Fix 2: Use fast internet connection
If you have a fast Internet connection, you can change your settings to make sure videos always play in HD qualities when they’re available:

1. Go to your Settings page
2. Select Always choose the best quality for my connection and player size
3. Click the Save button

Fix 3: Change cache settings
If this is actually happening to you quite often, the cause may be your own ''cache/memory'' which is filling up. That's especially true if you've seen a lot of videos or played a lot of games/music.

Exit out of YouTube (but not your browser) and do one of the steps below. Afterwards, shutdown and reboot. Not only will this "free up" your computer's memory, everything will be "fresh", and load faster.

Internet Explorer 8:
Tools → Delete Browsing History, Temporary Internet files, Cookies
Preserve Favorites (leave blank)

Google Chrome:
Tools (wrench) → Clear browsing data...

Fix 4: Stop other programs using Internet bandwidth
If you have other applications which are running at the same time, those will slow down YouTube too. If you have "Torrents", you might want to consider closing or uninstalling them.
Also, "Adblock Plus", various Instant Messengers and "video accelerator" are known to interfere with YouTube.
Another thing you may want to consider is turning off your anti-virus -- but only momentarily. As long as you remain on YouTube you will be okay, because you cannot get a virus from YouTube.

I hope this helps. If you still face any issue, you can revert back and I will provide further assistance to resolve your issue at the earliest.

Regards,
Lokesh