Likewise you also should be glad and rejoice with me. (Philippians 2:18, ESV)There is an interesting play on words here in the greek. In the previous verse, Paul states that he will "be glad and rejoice" with the Philippians. Now he tells the Philippians to "be glad and rejoice" with him.
In verse 17, "be glad" is χαίρω (chairō). Meaning, "to rejoice" and the tense of this verb is that it is happening right now. "Rejoice with all of you" is συγχαίρω (synchairō). Notice that it is basically the same word, with "syn" at the beginning, hence the "rejoice with." It is almost as if Paul was saying, "I am joyful and my joy is synchronized with yours."
Now, in verse 18, Paul restates these two verbs, with the slight change into imperatives for both. Now they are commands. As if he is now saying, "You should be joyful... choose to be joyful... and let's get it synced back up with my own joy!"
The application: Choose Joy!