In my submission records, I always say a poem is returned, never rejected. That helps to never think of my poems as being rejected. But not getting anything accepted from a submission can still sting, especially for those new to haiku. One way to lessen that sting is to write more. I don’t mean right after poems are returned but all the time. The more poems you write, the stricter you can be in selecting what you send out, meaning that they will be better poems and less likely to be returned. But more than that, your greater number of poems will reduce the “weight” on any individual poem.
I once had a poem returned by a major haiku journal, only to have that poem win the HSA’s Henderson Award, which is just one of several ways that some returns may have nothing to do with the quality of the poem. But if you repeatedly get returns, some humility and openness to learning more about haiku may be in order. Or maybe you’re sending senryu to a gendai haiku journal and need to read the submission guidelines more closely. No matter what, keep reading, keep writing, and keep submitting. And remember that publication is not the only way a poem can be validated.
Don’t take “rejection” too seriously. However, don’t take “acceptance” too seriously either. They are both subjective.