We just felt it was time to have another. Our son will be 2 in January, and our 2nd son is due the beginning of December. Many of my friends with multiple children say that having a second child wasn't that much more of a financial strain, esp. since they are close in age and can use the old crib, carseat, baby gear as the first.
I loved having a playmate as a child and a dear friend in my sister as an adult. My husband feels the same about his 3 siblings.
Every one has to make their personal decision, I feel that each additional child we bring into our home, it adds more happiness for our family overall. I know there will be times of stress and more financial hardship, but that's something we are willing to sacrifice in order to bring more love in our home. I personally would feel guilty to avoid having another child just so that I cna have more financial luxury, but that's just me!
I think if it is a decision you struggle with back and forth, than somewhere you really do want to add another member to your family.