A common response. And completely bollocks. You absolutely can know good and happiness and love without knowing pain and suffering. And, vice versa, you can know pain and suffering without knowing good. There are babies born with terrible genetic disorders and diseases who know nothing but pain for the short few hours or days they manage to keep breathing at the hospital, while the doctors desperately try to mend their little broken bodies. Are you seriously saying they don't know suffering just because they haven't known good? And the opposite is also true. Are you saying that a child growing up in a loving environment with caring parents isn't experiencing good, isn't experiencing love simply because he hasn't yet experienced terrible pain? That's ridiculous. Following such a ridiculous path of thought implies that we should tie children down and torture them, that way they can start to know good and happiness early on. It's absurd.
This kind of muddled thought comes from a misunderstanding of what experiencing pain does provide us. It provides us an additional experience - gratefulness. Gratefulness that we are no longer experiencing the suffering. Relief. It makes us aware that the good experiences we have in the future are not to be taken for granted (ie wisdom). BUT, and this is a very important but, that gratefulness that comes afterward doesn't (necessarily) justify the suffering from which it arises. If it did, then I could justify tying you down and torturing you - because the relief and gratefulness and wisdom that you would feel afterwards would make the horrible suffering you endured worth it. But we all know that's not true. It cannot be true - that's why gratefulness follows in the first place, why we're grateful to no longer experience the bad - because it's not worth it.
Now, I should qualify this by stating there are certainly some instances in which experiencing bad is worth it. For example, I consider the discomfort of working out justify the benefits that come afterward. That's not the kind of bad that requires "God to do some 'splaining". We're talking about the other kind, the senseless kind. The diseased, pain-wracked babies. Starvation. Torture. Dying in car accidents. Et cetera. How are they justifiable. (I'll give you a hint - they aren't)