I'm a mentor for students taking the Stoic Essentials Studies course at the College of Stoic Philosophers. Because this question keeps coming up with nearly all my students, I decided to capture my thoughts on this topic.
At some point, whether at the beginning of the course or some time during it, a student will express some version of this concern regarding the Stoic view on fate: it's hard to believe that everything happens for a reason.
Every time a student shares this concern, I share what A. A. Long wrote on the subject along with my perspective.
Long, who has perhaps studied Stoicism longer than anyone alive today, makes this observation:
"If Nature's providence is all-embracing then any event which causes injury or suffering has to be interpreted as something which, if all the facts were known, would be recognized as beneficial by rational men. As Pope, following Shaftesbury, wrote: 'All discord, harmony not understood, all partial evil, universal good.' But all the facts cannot be known and therefore the supposed value of much that happens must be taken on trust. This optimistic attitude towards natural events, no matter how terrible they may seem, is one of the least palatable features of Stoicism. It is one thing to say that human vision is limited, unable to grasp the full cosmic perspective. But even at its noblest, in the writings of Epictetus or Marcus Aurelius, there is something chilling and insensitive about the Stoic's faith that all will turn out well in the end. They were the only Greek philosophers who tried to find a rationale for everything within their concept of a perfect, all-embracing Nature" (170, emphasis added).
I don't have an answer for people who express this concern, as this aspect of Stoicism weighs on my mind too. The way I choose to look at fate is this: it is what it is and the sooner I can accept events as they are, the sooner I can pivot to focus on how I choose to react and move forward. Perhaps the Stoics belief in 'all will turn out well' is just a short cut to get to acceptance.
Long, A. A. Hellenistic Philosophy : Stoics, Epicureans, Sceptics. University Of California Press, 1986.