Different Types Of מצטער

The Rema (639:5) says one is obligated to eat the first night even in the rain. The Rema (640:4) says בלילה ראשונה אפי' מצטער חייב לאכול שם כזית in reference to the law that someone in pain is exempt from sukkah. Presumably this law is the same as the previous one, that the exemption of מצטער does not apply the first night. However, the sources listed as the source of the Rema are different, the law of rain is cited from Ran and Rosh and the law of  מצטער is cited from the Kol Bo. Why does he need a new source? The Sharey Teshuva cites from the Bircay Yosef and the Eliyahu Rabbah that the previous law of a sick person who is exempt may apply even on the first night. Why would it be different?

What we see are three different applications of the law of מצטער. The law of rain the Mishna describes as being like a master rejecting the service of his servant. There is room to argue that even when a person is not in a level of grave distress to the rain, he is exempt due to the siman that Hashme is not interested in his act of sitting in the sukkah (see Mishna Berurah 639:31) even if a person is not בצער if the food gets ruined one can go inside. That also explains why one doesn't have to go back out to the sukkah even if the rain stops (sif 7.) For the act of sitting in the sukkah is not desired. There, one could argue they must still eat in the sukkah on the first night when there is an obligation to eat in the sukkah. However, in a true state of צער on could have said there is an exemption even the first night. That is what the second Rema sees in the Kol Bo that there is an obligation even in that situation.   

The simple understanding from the Gemarah is that the reason מצטער is exempt from sukkah is due to the rule of תשבו כעין תדורו. However, the Ritva on daf 26 cites from the Ramban that the exemption is derived from the word אזרח that only one who is able to sit tranquility in the sukkah is called an אזרח. Based upon this we have two reasons to exempt someone who is מצטער from sukkah. One reason is from תשבו, that the sukkah itself is disqualified while the reason of the Ritva is an exemption on the גברא - that a person who is not settled is exempt from sukkah. 

The Taz (640:8) differentiates between one who is pain due to being in the sukkah who is only patur when his pain is alleviated by leaving the sukkah vs. someone who is in a constant state of pain such as a חולה who is exempt from sukkah even when being out of the sukkah will not help manage the pain at all. What is the difference? In the case of the the constant pain, there is the petur of תשבו, the person can't be in the proper state of mind to do an act of sitting in a sukkah. However, an external cause of distress and pain only exempts the person due to האזרח, since he can't be in a state of tranquility but it is an act of sitting in sukkah and the exemption only allows for one to find a better area to sit. 

In light of this, there may be room to differentiate between the exemption of the מצטער which a personal feeling, and the exemption is due to אזרח, which will not be applied the first night when one is obligated to eat in a sukkah even in a situation of pain. However, a sick person is exempt due to תשבו for their sitting in sukkah is not an act of ישיבת סוכה since people in constant pain can not have an experience of כעין תדורו and one might say since it is not an act of ישיבת סוכה, the exemption would apply even on the first night (Birchas Mordechai.)