Doubting the filching age will steal his treasure; (6)
Here we see the problem with the miser: he can't show off his wealth without fearing that other people are going to steal it. (Remember that "Doubting" in this context means "fearing.") But what good is money if you can't spend it? That's what it's made for, right? Not for the miser, it isn't—and thus he spends his life devoted to something useless. Now we can see how wealth is worst for the person who loves it best.
Now counting best to be with you alone, (7)
Notice in this line how Shakespeare subtly shifts from making the speaker talk about the miser to making him talk about himself—by sticking in the word "you." But, even so, he keeps things in the key of miser, if you like, by using the word "counting" to express the speaker's thought process. Shakespeare could just as well have used some other word, like "deeming" or "thinking," couldn't he? By choosing this word, he suggests a finicky, mechanical decision process, a thinking process less suited to love of people than love of money.
Then bettered that the world may see my pleasure; (8)
The same goes for this line as it does for the previous line. Remember that, in the previous line, the word "counting" appeared in the phrase "counting best." Now, by adding the word "bettered," it makes us think of a miser who, having tallied up all his money, suddenly realizes that there was a whole additional stack of coins he forgot to account for. This numerical way of thinking doesn't seem appropriate to love, but rather suggests the way the speaker's thinking has become perverted by greed.