[The following reflections were prompted by a very interesting conversation with a young scholar, Johan Olsthoorn.--ES]
The official Letters by and to Spinoza that were collected in his posthumous works start with a letter from a very important person (The Secretary of the Royal Society, Oldenburgh), and that correspondence segue-ways nicely into a correspondence with Boyle--just about the most famous living scholar of the period. It would be a very surprising if this was the very first letter ever addresed to Spinoza. The Letters also cover only the last sixteen years of Spinoza, life. Spinoza was not very old when he died, but even so it implies that during the first twenty-eight years of his life the mailman never knocked at Spinoza's abode. (That's not a miracle; mail was expensive. But it was also not so uncommon in seventeenth century Holland.) From a note on the back of a letter (Letter 28 to Bouwmeester) that was not included in the original collection (but published later in Van Vloten) we know that not all letters known in 1677 were included in the original collection. It is safe to say that these letters are very much a biased selection.
Even so, even including the letters found and collected since the publication of Spinoza's posthumous works, we average less than six letters per year during the last sixteen years of Spinoza's life. That's not a lot. (Still no letters have been found, I think, that predate the first letter from Oldenburgh.) Moreover, with one exception, none of the letters from Spinoza initiate a correspondence. In a few cases Spinoza's is the first letter extant from a correspondence, but in all of these Spinoza is responding to a now lost letter. In these generalizations I leave aside a very brief note to Prof. Graevius in Utrecht (in which Spinoza wants a letter (presumably written by neither Graevius nor Spinzoa) returned so he (Spinoza) can return it to its rightful owner). [I am not claiming this note isn't interesting for the light it sheds on Spinoza's network in the academy!]
We can infer from these letters that Spinoza was remarkably self-sufficient intellectually. He did not seek out others in the Republic of Letters, nor did he sought to fashion himself through these letters. He also does not seem to be very curious about what others are up to in this Republic.
Okay, what to make of this? Despite the biased sample, I infer that Spinoza avoided intellectual situations in which his self-sufficiency was at risk. (This is not to deny that we have a few instances that during a correspondence he would show curiosity about events or people beyond the correspondence.) Now, this illuminates what Spinoza means when in the Ethics, he claims that the "free man tries to establish friendship with others." (EIVp70) and avoids receiving favors from then. This kind of friendship is a withdrawal from any kind of commerce, in particular, there should be no give and take. Now in context it is clear that the un-free people have -- whatever they may think about it -- nothing of (fundamental) value to offer to the free man. (Spinoza suggests the free man should withdraw from such commerce rather than try to keep book by some double account in which the value of favors is treated twice over--one according to his own standards and the other according to the vulgar.) Now, in light of Spinoza's own practice, it should come as no surprise that according to EIVp70 the free man "tries to guide others" (and himself according to the judgments of reason). Here I am going to ignore the very non-trivial problem how Spinoza secures that the guidance of reason, whose deliverances have to be univocal (that is needed in his political philosophy), is both determinate and sensitive to local context.
Of course, the only genuinely self-sufficient entity in Spinoza's universe is God/Causa Sui.
Now,the main (if not only) exception to my generalization, is, in fact, that found letter 28 to Bouwmeester. This letter suggests that Bouwmeester and Spinoza had once jointly planned a correspondence, but that Spinoza was impatiently waiting for Bouwmeester to initiate. So, why did Spinoza (against the pattern of his life) reach out to Bouwmeester? Well, he wanted Bouwmeester, a physician, to send him a conserve of red roses. As Michael Morgan points out in his footnote to Shirley's translation of the Letters, "It was held that a conserve of red roses is remedial for diseases of the lungs, he probably prescribed this remedy to Spinoza. Note that this letter is the earliest indication we have of the tuberculos which eventually killed Spinoza." It is physical need that makes Spinoza reach out just this once to another (in defiance of the norm that he probably lives by).
If Spinoza ever felt a need to philosophize with others his devoted editors and he tried hard to ensure that posteriority would never find out. It is not unlikely, I think, that he managed to suppress expression of the need if it arose in him ever.