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Hey Schuyler, that does seem strange!
I suppose he could have argued that, even if you feel happier on such occasions, it's not true that you are happier. Yet he doesn't seem to draw a contrast between psychological happiness and actual happiness, between your subjective experience and your objective state of human flourishing (as the Aristotelians might put it). Instead, as you've noted, he says that if you have acquired what is natural and necessary then you're as happy as you can possibly be.
This line of thinking leads to the importance of thinking through what is indeed natural and necessary:
As soon as the pain produced by the lack of something is removed, pleasure in the flesh is not increased but only embellished. Yet the limit of enjoyment in the mind is produced by thinking through these very things and similar things, which once provoked the greatest fears in the mind. (Principal Doctrine 18)
The flesh assumes that the limits of joy are infinite, and that infinite joy can be produced only through infinite time. But the mind, thinking through the goal and limits of the flesh and dissolving fears about eternity, produces a complete way of life and therefore has no need of infinite time; yet the mind does not flee from joy, nor when events cause it to exit from life does it look back as if it has missed any aspect of the best life. (Principal Doctrine 20)
You must reflect on the fundamental goal and everything that is clear, to which opinions are referred; if you do not, all will be full of trouble and confusion. (Principal Doctrine 22)
Insofar as you forget nature, you will find yourself in trouble and create for yourself endless fears and desires. (Fragment 203)
I suppose that to modern ears his advice might sound like hopeless intellectualism: you can think your way to happiness. On the other hand, perhaps such a recommendation wasn't so far-fetched to the founder of a philosophy!
—Paul
Next: Letter Seven: Philosophy as Medicine