Betas are awesome (well, they can be) - it's such a boost to see that someone's laughing in the right spots and reacting the way you wanted the reader to do. Also, they can point out where you're unclear and confusing them, which is very helpful. You want to find someone whose attitude is "I love what you're doing -- and you know what, I think it could be even better in these spots".
You do have to go into it understanding that you're not already perfect, though. Your beta is there to help you improve, but that involves criticism, and criticism can be tough to take if you're not prepared to handle it.
At the same time, remember that your beta is not God and they're not always right

It's your story. Consider their suggestions, but feel free to ignore if they're not what you want the story to do. It's especially helpful if you can articulate *why* the suggestions don't work - you can understand more about what you're trying to do. Knowing when something feels right is good, knowing why it feels right is better

IMHO.
PJ
who has learned soooo much from beta readers and other FOLCs through the years - this fandom is amazing.