Godparenting

My sister has asked me to be her son's godmother and I'm ecstatic! Actually, I was kind of expecting it but it felt even more exciting when she finally asked me.
I didn't realize we had to attend a "discernment process" or what they often call a Baptismal Preparation seminar to become godparents. I've been a godparent before--to four other kids to be exact, but I've never been required to attend a seminar with any one of them.
In the past I was told that when asked to become one, you can't say no. If you're a parent and someone wants to be a godparent to your kid, you can't say no either. But why not? What does it really mean to become a godparent? To me, accepting an invitation to be a godparent means accepting a massive responsibility--responsibility to support and help the parents as they raise the child, responsibility to guide the child as he/she journeys through life--through Christian life, and responsibility to be there for the child when he/she is in need. Having said these, I find it perfectly OK to decline an invitation to become a godparent if I'm not ready to fulfill these responsibilities towards the child.
Of course, with my nephew I am ready and I am deeply honored to have been given the task.
The seminar that we attended today confirmed my beliefs about godparenting--or at least most of it. I am happy we went through it because it made me think about what I have done for my godchildren and whether or not I have carried out my responsibilities towards them. But as a future parent, it also made me contemplate about how to choose my child's godparents--who do I give the responsibility to and will they take it on? I grew up not knowing any one of my godparents, so I guess my parents didn't choose very well. I wish they did, now that I know what it really means.