Sonnet 75 portrays jealousy as a direct result of pride and the inability to be satisfied. If lovers and misers could be content with the company of the person they love, or with their money, there would be no problem. Easy peasy. Unfortunately, neither of them can be satisfied with that: their pride makes them want to show off what they have to the world. As soon as they do that, however, they start worrying that other people will steal what they have. Of course, if they were truly confident that the person they loved loved them back, they wouldn't have to worry about that, right? So, jealousy may not be the source of the speaker's problems, but it's definitely part of it. What makes jealousy especially bad is it tends to start the cycle all over again. By being jealous, the speaker gets even less confident, which makes him want to show off even more, and so on. It can't be long before the speaker and his beloved find themselves on a TV talk show hashing out their problems.
Lines 11-12 suggest that the speaker and "you" don't have a very strong relationship. Bummer. Perhaps if they had a stronger relationship, the speaker wouldn't be so jealous.
Now hold on a minute. The poem doesn't give us any reason to think that "you" might be jealous of the speaker. In fact, given that the speaker seems to be wasting away with desire for "you" (and thus not seeing anybody else), this seems highly unlikely.