A lot of it has to do with geography. Poland sits almost in the middle of Europe, with few geographical features protecting it. That means Poland can be invaded from any direction, particularly since for much of Poland's history, Poland had powerful neighbors on its borders.

The second reason has to do with the Polish state itself. There was a time when Poland was a great power in its own right 16th-early 17th centuries. However after that Poland went into decline. It lacked central authority to the extend that its neighbors had, which made it difficult to organize an effective defense against invaders. The King's power was limited. In addition, in the Polish legislative body, any member could effectively veto legislation, which made it not only ineffective but also a liability that Poland's neighbors could abuse. Lastly, Poland was quite heterogeneous in its demographics, and Poland's enemies were able exploit division within the country. After its decline, Poland was no longer able to defend itself against powerful neighbors neither politically nor militarily.

In the 20th century, the Polish state was restored, and it even enjoy a modicum of power. However, that was only the case because its two most powerful neighbors - Russia and Germany - were greatly weakened by WWI. But fundamentally, things remained largely unchanged - Poland was a small country (only a shadow of what it was back in the 16th century), flanked by two large and very powerful nations. Instead of allying itself with either one or the other, Poland's leaders decided it would be best to ally with France and Great Britain, neither of which could really defend it. Thus Poland was invaded yet again.

Reason is geography. If You look at world map You'll see almost all countries in world have borders on natural obstacles, and where there are no obstacles if You look back in 100 years history - there were conflicts. Poland is great country if You live in 10 century or 15 century - that's why it got so big in those times. Flat plains, amazing amount of land to grow crops. Enough water for normal farming, no natural disasters beside floods, which actually are good for farming, no obstacles in transporting of foods. Much better than for example Austria or Switzerland, or south Germany. But for same reasons Poland is extremely easy to invade. If You want to go from Germany to east - go through Poland. If U want to go west from Russia - Poland is your choice. So Poland's only chance was to be so strong so it would be impossible to invade it. It worked for some time. Poland even invaded Russia and polish king ruled in Moscow for few years, but in the end its impossible to fight vs Russia Germany Sweden Austria and from time to time Ottomans. So geography made Poland a battlefield for 400 years.