It is not very difficult to mourn with those who mourn. It is proportionally easy to feel the loss of a sorrowful person and to express real sadness for their loss.
Job 2:11-13 let us know that it is a part of human nature to mourn with the people who mourn. There're even times when you don't personally know the fellow who has encountered a great loss, but after hearing their story, you become very sad.
However, it's also part of human nature to be selfish and an ordinary man is very pleased so far no one around him gets anything better than what he has. Every individual on earth wants the best for himself or herself.
When that best we are talking about goes to another person else, it becomes very difficult to be truly happy for that person. This is a very terrible human attribute that every Christian must not permit in his or her life.
The nature of God that we bear does not have such character traits. His instruction to us in Romans 12:15 plainly shows what He wants of us in such situations, and we should take it very seriously.
How can we rejoice with the people who rejoice? First, you have to cease likening yourself with others. The moment you begin looking at others people around you and comparing what they possessed with what you have, you will soon encounter destruction.
In Matthew 20:1-16, Jesus told a very intriguing story of different men hired at different times of the day. The men who were hired initially were very pleased with being paid a penny each.
However, at the end of the workday, they were not happy when they got this exact amount. Why? It was due to the fact that they were comparing themselves with the other hired men who had worked for a shorter period and received the same money as them.
The master who hired them told them to take what had been given to them and go their own way, rather than putting their attention negatively on what he had chosen to do for the others. Secondly, pray for the people you envy.
When you wholeheartedly pray for someone always and long enough, it becomes difficult to nurse any hard feelings towards them.
Finally, be grateful at all times. Always remember that there are people in worse circumstances compared to you; therefore be thankful and content always.