Waiting for reactions

As I'm actively looking for a new job, during the last few weeks, I've send out some applications already. But after sending them, comes a very boring and nerve-wrecking stage : waiting for reactions.

Some companies have a web application or a dedicated E-mail address for sending applications. Other have a good policy on handling applications. Both result in a (semi-)automatic answer, thanking you for your application, confirming your application was received or instructions on when to expect an answer (or not), or what to do when you haven't received an answer within a certain period of time.

Others send out letters or E-mails thanking you for your application, but regret to have to inform the vacancy is already filled or you don't seem to satisfy all the criteria needed for the job, wishing you good look on finding a job.
But, in case you're not suitable for the job, they never tell you why you're not fitting in. Was it something in the presentation letter? Was I too modest, or not elaborative enough? Did I write something offending? Didn't I apply to all courtesy and protocol which is needed in writing a presentation letter?
Or do I miss some experience or education? Did they put more value in the optional prerequisites, than was laid out in the vacancy?
I probably will never know, but it keeps me wondering.

But some don't react to your application at all, which is actually quite rude. If they would just send a short message informing you don't fit in (like I mentioned in the previous paragraph), you can stop waiting for an answer. But if no reaction comes, when can you stop hoping for a reaction?

But fortunately, you only need one job, so if you send out enough applications, some reach companies of the first type, resulting hopefully in a positive reaction or an invitation for further testing or an interview. And once you have a job, it doesn't matter some applications never get answered. But while you're waiting, it can be quite stressful.