The first book must be able to stand alone, with no dangling threads; the reader must have a satisfying experience or there will be no sequels. This is more true for unpublished authors.
I don't regret writing my sequels, because that's where all my story energy wanted to go, and I was able to write other things once I got it out of my head. I didn't write sequels to two of my other stories, which I also hope to make a series, because I knew it didn't make any sense to do that, and I didn't feel that compulsion to write them. Also, I know my characters and their world inside and out now, and I find it reassuring that even if much of the actual writing in the sequel doesn't get used, I know that there's plenty of story left to tell. If the first book sells and sells well, I have plenty more to say.