Dear Friend,
Incarnation into physical existence is the quickest way to evolve spiritually. This is the reason for physical existence. However, we must not forget that we have free will. On transition from the physical to the spiritual world we enter a multi step process. The first step is one of rest and recouperation. If you think of reincarnation as a school this step is the end of term break. Because of free will we can take as long as we like resting.
The next step is one of reflection where we can reflect on the last physical life we experienced and learn from our mistakes.
After this we can progress from the spiritual "mental world" to the spiritual "higher mental" level. At this point we can decide to reincarnate and also decide the general experiences we will be subjected to. Because there is no such thing as time or space in the spiritual levels, we can wait untill we feel we are ready to progress at each stage of the journey. Of course dear friend this is my understanding of the process, which does not mean it is an indisputable fact.
Peace be with you.