When something exciting happens in your life, I'm sure you can't wait to share it with others, right? But why? It's usually because you want them to share in your excitement; you want them to experience the same emotions you are experiencing.
Be happy with those who are happy, and weep with those who weep. (Romans 12:15, NLT)
The above scripture commands us to show empathy. Empathy is the ability to understand and share the feelings of others. It not only applies to understanding how someone might feel when they are going through hard times, but also when they are rejoicing. It is important as believers to share in the joys of our brethren. We are all connected through Christ and so we no longer have to feel jealous or envious of another person's joys.
If one part suffers, all the parts suffer with it, and if one part is honored, all the parts are glad. (1 Corinthians 12:26, NLT)
In Christ we are one. Our lives are no longer just our lives, but they belong to Christ and everything we do has an impact on the rest of the body whether directly or indirectly. When we don't realise this, we may see another person's success as a "threat" to take away / replace what we are doing or an attempt to make us look small / inferior. However, when we recognise that another person's joy is also ours we receive the strength and encouragement that comes from sharing in their joy. We are able to understand what God has done for that person and recognise that this means He can also do the same for us.