Im sorry, i know this is too late to comment but i just want to put my personal opinion. Most people think that this song has no meaning but actually it holds very deep meaning no matter how you interpret it. Some people say that this song is for fans but i personally thinkg its for the ex members, this is why:

The whale: If you serach online for the meaning of whale totem you will see that it defines good luck and long lasting love.

Basketball: In exo showtime Luhan was known to be an excellent basketball player hence this might describe him and sportmanship.

Fighting: Martial arts points to Tao however seeing that the members who are fighting is Suho ( leader, responsible and power is water) and Chanyeol ( Power is fire to descrive anger) it might be showing that they are fighting over something.

Galaxy: if you mention Galaxy to exo l they will think of Kris as he really loves the galaxy.

Song interpretation (personal point of view):
This song is saying that the ex members as the whale were being sent off with best wishes anf long lasting love from the members. At some point it showed that the members were upset and lonely that is to describe how they felt when the members just left. Kai dancing showed pain as it seemed he was fighting an invisible force like memories is what is hurting him. Suho and Chanyeol fighting Chanyeol power is fire so it shows that he is angry and is determined to get the ex members back while Suho is trying to make him let go of them. When the two of them lie down together it shows that no matter what difficulties they pass, their friendship will always be the first thing they consider. Happy moments in the car represents their happy times together. The lyrics itself is saying that they wish to be able to sing to the members again as one to go through things together again and relive all their memories together again.

(i know that my comment is very messy but i hope that you get the general meaning, i also cant explain my thoughts in deep details here but if you have questions please feel free to write under my comment)

A lot of people say the whale and the astronaut/space concept did not make sense in the Sing For You MV, but it actually has a lot of significance.
The whale concept was derived from the a whale called Alice aka ‘The Loneliest Whale in the World’, who is an unidentified whale species, being the only whale emitting a call at 52-hertz. 

The astronaut concept was derived from the story of the astronaut who spent 48 minutes in his spaceship and coming back saying that those 48 minutes were the longest and most loneliest of mankind.

The combination of these two concepts placed within the MV discusses being away from home, being somewhere unfamiliar (since the whale and astronaut is in space) and being isolated. Isolation was one of the themes in the mv (when Kai was seen being alone when the other members were playing/when Chanyeol and Suho were fighting).

The song and the MV seems to talk about how everyone, in their lifetime, would meet a low point where they feel most isolated and most out of their place, but that this song (and EXO) will be there to warm their hearts.
In a way, this song could also be indirectly dedicated to the members who have left considering the lyrics of the song. (ctto)

i do agree to what you say...even though i'm an exo-l
i first encounter exo is through their songs...yes...i'm those who will listen first before checking out who is the singer/band/group...
okayy...most of k-pop boy groups are handsome guys...that i can't deny it...
for their new song, i like it...but for the mv part....i'm like what is the message they are trying to tell us the viewers ? ....okayy..you guys can laugh at me...but the truth is i really don't understand the hidden meaning...