And for the peace of you I hold such strife,As 'twixt a miser and his wealth is found; (3-4)
When we first read these lines, we aren't likely to think of them as talking about jealousy. Why not? Because they don't say anything about jealousy specifically. So why are we talking about them under this theme? Because they describe what jealousy feels like. Think about it: line 3 says that the speaker is experiencing "strife." At this point, we're probably thinking, what kind of "strife"? Then line 4 tells us: the kind that "'twixt a miser and his wealth is found." Fair enough—but what kind of "strife" is that? The answer to this question comes in lines 5-6, where we learn that the miser can never enjoy his money because he is afraid that other people will steal it. And if you're afraid that other people will steal your money, that means you're jealous of them, right? (This last point is easier to see if you bear in mind that all this stuff about the miser is an extended metaphor for the speaker's relationship with his beloved.)
It's obvious that, if you're afraid other people will steal your significant other, your feeling is one of jealousy. So, the miser's problem is one of jealousy. But what if you then asked how that jealousy makes the miser feel? The answer to this question—-"strife"—takes us right back to where we started: lines 3-4. Thus, even though it doesn't look like it when you first read them, these lines are actually the start of the speaker's reflection on the theme of jealousy.
Now proud as an enjoyer, and anon (5)
Once again, this isn't exactly touching on jealousy as such, but it's definitely a related issue. When the speaker describes the miser (and, metaphorically, himself) as being "proud" because we gets to "enjoy" his possessions, that shows that he doesn't enjoy what he has for its own sake. Wouldn't you agree that pride is an emotion associated with what other people think of you? And if your feelings of love are wrapped up with your thoughts about other people's opinions of you, it seems like only a couple of steps away to start worrying about what other people might be thinking and feeling about the person you love. This moment thus provides one of the poem's many warning signs illustrating the problems with the speaker and his beloved's relationship.
Doubting the filching age will steal his treasure; (6)
Drumroll please! And now, ladies and gentlemen, it is Shmoop's great pleasure to present to you… the Theme of Jealousy! Ta-da! Yes, here it is. The speaker, talking about himself through the metaphor of the miser, is afraid that people will steal his beloved. Case closed.