Because one cannot do otherwise, of course. But more seriously, I have been thinking about this question in the case of Spinoza. It’s clear that he took himself to be a necessitarian — “I have shown clearly enough that from God’s supreme power all things have necessarily flowed by the same necessity and in the same way as from the nature of a triangle it follows that its angles are equal to two right angles.” He never flinches from the label, though many of his correspondents pester him with all of the ugly consequences this view entails (no contra-causal freedom, seemingly no moral responsibility, sin backsplashing on God, etc). The problem is that, throughout his works, he never completely explains exactly how God’s nature is supposed to necessitate all things, and even goes so far as to demonstrate that from God’s nature alone you cannot derive the existence of any particular finite thing. So it’s puzzling why he should insist on being a necessitarian while at the same time not buying the whole hog and making each and every single thing flow out of God somehow.
I don’t really believe that someone becomes a necessitarian solely from philosophical arguments. There has to be something else attractive in it. So what is it? Is it that you can forgive yourself of all the stupid and mean things you’ve done? Is it so that you can comfort yourself for all the things and people you’ve lost, since you know that nothing could have been otherwise? Or is it an aesthetic attraction: that whatever makes people love math or geometry also makes them want to see it in the cosmos as a whole? I tend to think that is what pushed Spinoza in this direction. Good lord, no one (except maybe Euclid himself) can imagine loving geometrical proofs more than he did.
Is it true that anyone who is drawn to necessitarian is also drawn in the direction of Stoicism? What might this imply? Is it a kind of fear of the emotions, and the compromising positions they can put us in?
I think what Sp really loved about the geometrical method was its mechanical necessity and unshakeable firmness. And why love that? Maybe because he saw around him so many insistent intellectuals who disagreed violently with one another — the Jews, the Calvinists, the Catholics, the Remonstrants, and so on. Maybe he thought that by taking on the geometrical form he could secure, if only for himself, a set of beliefs which would stay strong against all attacks. Temperamentally, he couldn’t handle the rough-and-tumble marketplace of ideas — which, in the 17th C., was far rougher and tumblier than any intellectual sphere is today. Maybe his adherence to the form, and his attraction to Stoicism, both stemmed from a fear of other people, and the ways they can truly mess you up.
Here’s Nz’s psychoanalysis of Sp’s attraction to the geometrical form in which he cast the Ethics (BGE 5):
… that hocus-pocus of mathematical form in which, as if in iron, Spinoza encased and masked his philosophy — ‘the love of his wisdom’, to render that word fairly and squarely — so as to strike terror into the heart of any assailant who should dare to glance at that invincible maiden and Pallas Athene — how much personal timidity and vulnerability this masquerade of a sick recluse betrays!