Is there any reasons why the background of iPhone (iOS) text messages is gray (for messages sent from others) and green (sent by myself)?
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I'm not part of Apples design team, so I can only make an educated guess. The color green for your messages (or blue if there are iMessages) and gray for the opposite messenger doesn't have a meaning itself. However, differenciating colors in a chat or messaging is there for a reason. From a UX perspective it should be easy for you to know which party said what. There are three more ques to that: the alignment of the message, the margin and the callout pointer:
The opponents message is left aligned, pointing to the left edge and having a margin to the right as opposed to your messages which have the opposite. All of this means that you are supposed to clearly see the conversation in messages.
This gets pretty abstract but if you take all other factors out - alignment and so on so that the only differentiation is just color, then think about what makes sense to you.
There is the subdued color (gray) and the louder color (blue). There are two actions in this scenario- sending messages and receiving (passive). Why would you want to associate the louder color with the less consequential action?
Sending a text is more consequential then receiving- you can type whatever you want but you have no idea what to expect- so you take more ownership in the texts you send. The louder color then indicates that these items belong to you, and that the gray messages are secondary, a response to the message you send (which most of the time they are), because they are not indeed yours.
That's why when looking at a random image of texts (for example the one in Benny's answer), you will gravitate to the louder color (blue) first and assume that's your end, because you have more of a 'stake' on this side of the convo. You'll then realize the actual colors are not as important as the hues, what pops out more (come into the foreground: reds, brighter colors,etc) vs what recedes more (grays, cooler colors,etc)