Rejoicing with others is one of the most difficult things to do especially when what is being rejoiced about is what you really need at that point, but it doesn't seems coming forth.  Just imagine you and your friends  wrote  an examination, and you are the only one that didn't pass. What would be your reaction? Would you find it easy to take joy in the success of your friends? 
Some other time, you might even pretend to be joyful, but you are actually bittered within. Then, you will start questioning God - What have I done to deserve this? Is this the best way to reward me ? Am I not serving You? Does he/she has two heads? Why am l always unfortunate with You?

Meanwhile, all this is the beginning of loosing one's peace of mind. When you find it difficult to be joyful, then you are in for the opposite. Some will even go to the extreme by spreading rumours and lies about those who succeed around them. Does it really count? Will that make you also get whatever you want? Most times, it always hinders getting one's blessings too.

Hence, there is a need to find joy in the joy of others. As said earlier, it is a very difficult task, therefore, it requires learning everyday for it to become easier.
You need to understand that for your neighbour to succeed, then there is a high tendency that you too can. Do not forget that God can never run out of blessings. For God to bless others does not mean He is not ready to bless you too. There is this saying that when God bless your neighbour, it means that He is in your neighborhood. It is an assurance that indeed your time is coming.

Also, do well to encourage others on their achievements. For instance, if your colleague just get promoted, you can as well encourage them that this is the time to do even better. In doing this, you are also rejoicing with them. You never know whether their success will pave way for yours also. 

Choose to rejoice in others' goodness, and watch out for what is in store for you.