3 sentences

1. 这就是我之所以这样做的原因。

   zhè jiù shì wǒ zhī​suǒ​yǐ zhè​yàng zuò de yuán​yīn.

   This is the reason why I did it.

   Source: Dong Chinese Dictionary

2. 他之所以迟到，是因为路上堵车了。

   tā zhī​suǒ​yǐ chí​dào, shì yīn​wèi lù​shang dǔ​chē le.

   The reason he was late was because of a traffic jam.

   Source: Dong Chinese Dictionary

3. 他之所以这么做，其实是有难言的苦衷。

   tā zhī​suǒ​yǐ zhè​me zuò, qí​shí shì yǒu nán​yán de kǔ​zhōng.

   The reason he did this was actually because he had some unspeakable difficulties.

   Source: Dong Chinese Dictionary