Rejoice with those who rejoice [sharing others' joy], and weep with those who weep [sharing others' grief].
Romans 12:15 (Amplified Version)
The above verse is about empathy - not to be confused with sympathy.
The Holy Spirit gives the Believer a special provision to obey the Word of God. Those who agree with God are able to know what others are experiencing and are able to bear that burden with the one who is suffering.
The problem comes when Christians are so sympathetic that they become useless. Sympathy is the beginning of awareness of the plight of another, but when one is not in fellowship with the Lord, sympathy becomes sentimentality. The sentimental one begins to feel what they think they would feel if they were in the situation, and it has little to do with the one actually going through the hardship. The sentimental one begins to suffer a blight on the personality because they have taken on an anxiety and a pain that is fantasy and not real.
The pitfall is deep and immediate for those who are faking the Christian walk.
Soldier of The Cross, this is about following hard after Jesus. This is about taking up The Cross. This is about the death of the flesh. This is about dropping the things that would encumber - not picking them up and adopting them as your own.
Travel light - serve the King of Kings.