I've written at length about this before on several occasions. The very short answer is "They had no other choice". The Swedish army during this era fought every single major battle outnumbered to some degree. At best they were "only" out-numbered 1,2-1 or so, at worst much more. To make the situation worse, Sweden always had a small population compared to her enemies and simply could not replace losses to the same degree larger nations could. Consequently, they had to win every single battle decisively or they would quickly be worn down and lose to the superior ability of larger nations to replace losses.

I highly recommend you take a look at this thread with answers by me and /u/vonadler, there is little in your question you won't find answered there. If you still have questions after that, feel free to hit me up.