Believe it or not, secondary infertility - defined as "difficulty conceiving after already having conceived and carried a normal pregnancy" - is often just as difficult and challenging as primary infertility.
Basically, you have a baby or child, but you can't get pregnant. Since you were able to get pregnant before, people naturally assume that whatever fertility challenge you had in the past is now behind you and that all you need now is to relax and the next baby will come along.
Women or couples who suffered both primary and secondary infertility know that this isn't always so. Think of a woman who has no fallopian tubes or a couple where the husband suffers azoospermia (absence of live spermatozoa in the semen). They're not going to get pregnant by relaxing. The fact that they were able to conceive before doesn't mean it's going to be any easier this time.
Comments like, "but you already have a baby, so it's not such a big deal" aren't really very helpful. When you decide that you really want another baby, it doesn't really matter if it's your first, second, third or even fourth - and maybe even beyond that. When it's not working, it hurts. When it's been difficult before, you're afraid it will be difficult again. You're even worried you'll never have a baby again. If you went through years of infertility in the past and you started trying again now, you wonder how much time is still ahead where you'll be fighting this battle... and infertility drains so much time and energy...
A word of wisdom to those who have friends, co-workers or children suffering secondary infertility: It's not very different from the first time. It's painful, it's scary and unfortunately, it doesn't always have a happy ending.