Why did paul commend on the virtue of giving as shown by the Philipians?

When Paul was preaching the gospel of God to the people of
Thessalonica, the Philippians sent him money for his needs on two occasions.
This, the Philippians did to reduce his suffering in prison. Paul wrote through
Epaphroditus to commend them for their kind gesture. He expressed his gratitude
for the aid sent by the Philippians and for their concern for him.

Paul in his own way appealed to Christians to give generously as
the people of Macedonia did. He showed his appreciation for what the
Philippians did by writing to thank them. He also made us to understand that
every person should be rewarded according to his manner of giving and that God
always help those who give generously.

Situations when a Christian should give are: 

1. When a neighbour
   is in need 
2. He should give when he is not expecting anything in
   return.
3. A Christian should donate generously to the development of the
   society, e.g. during building of schools, hospitals, town halls, churches, etc. 
4. A Christian should visit motherless homes and give them gifts.