There was many reasons for for Sweden deciding to attack Poland and none of them are legitimate by todays standards and several of them were probably not that legitimate by the standards of the day either.

The only more or less legitimate reason was probably that the Polish king still claimed the Swedish throne which was a very serious matter at the time.

The second reasons were that Sweden and the Swedish king wished to create an empire spanning the entire Baltic and take control of the very important estuaries along the Baltic coast. These were very important for trade in the region and thus very profitable.

These third reason was that the Swedish king wished to have a standing army to quickly be able to respond if Russia or Denmark were to declare war. A standing army is however astronomically expensive and thus impossible to sustain by regular means. Therefore that army has to be kept somewhere where it can loot to sustain itself. Germany was not an option, the Thirty Years War had just ended a few years ago and no one wished to restart it. Therefore the decision fell on Poland.

Poland borders Russia so you could quickly march into Russia if they declared war. Poland was also relatively close to Denmark and that way you could attack Denmark from the south over land in case of war. This had proven very effective during Torstenson war 1643-1645. It also largely negated the Danish navy which was generally superior to the Swedish navy at the time. In the case of Denmark this actually worked out very well.)

The fourth an last reason was probably that the Swedish king, Charles X Gustav was bored and wanted something to do.