Well, the reasons I doubt it are as follows.
1. Nearly every published line of Shakespeare was written in iambic pentameter--- this alleged quote is not.
2. Shakespeare would almost certainly have used thee, thou, thine, etc, instead of you and your.
3. I would bet that none of the websites that claim this is from Shakespeare will tell us exactly where it is to be found--- which sonnet, or what act and scene from which play.
4. The only (unlikely) possibility I can think of is that it is a thought of Shakespeare's, but re-written in modern language, without preserving the poetic meter. It simply doesn't sound like Shakespeare as it stands. That's why I would like to see exactly where it supposedly comes from, as I said in (3) above.