I've lately become interested in the wars of the 1600s-1700s and one thing I've been wondering is how did the European monarchs of time frame their expansionist behaviors, to their people and to themselves? I'm sure this answer may vary across time and country, but what were the most popular explanations? Was it for their own personal gain, in which case how did the people respond to being sent off to war? Was there some form of proto-nationalism where they hoped the country as a whole would gain from these wars? Did the ruler expect this to happen?
The attacks against the Turks were a very popular reason especially for rulers of the Habsburg monarchy and the Russian empire. These two are out of the time period you specified but Napoleon used national pride and zeal gained after the French Revolution and Bismarck fabricated threats against Prussia so he could start a "defensive" war.
Also breaking a treaty would require retaliation which would often lead to conquering a bigger chunk of land than you were entitled, possible looming threats (Austrian annexation of Bosnia), religious reasons basically anything, as is the case today.
A reply to /u/66Milyon
For that time period, religion was a major rationale for war. This includes both the Christians vs. Ottomans wars and Catholics vs. other Christians wars.
As far as "people being sent off to war" is concerned, this is a difficult topic. In some parts, a sort of national identity had started to develop, in particular in Spain. But this wasn't the case in Germany, the venue for the Thirty Years' War. During which time, the two religious wars became rather convoluted.
Inheritance was another major reason. Spain's continuing Eighty Years' War was originally due to Charles V's inheritance.
I'm happy to give more detailed answer if you ask a more detailed question. A comprehensive overview is a bit much to expect.