This widow had every reason to say no to Elijah. She didn't have enough to feed herself and her son. She had given up all hope. She was ready to die. But she said yes in her need and God provided for her abundantly. He provided food and drink for her and her son and she even was helped emotionally. Elijah was there then to at least say hello to her and talk to her occasionally. The people in her town had refused to help her and probably shunned her for they were not looking out for her.
If an opportunity comes to us in our need to help someone else, it is a good thing to say yes. We never know what God may do through our generosity. He can provide. He can increase our joy. So much -- there are no limits to what God can do with a small act of faith on our part.
One other point. I take this from Jean E. Syswerda's Bible study I've quoted in the past. It is surely something we should examine in our own lives. Are we taking care of the needy, the widows and orphans in our society who are living near us? And not just financially -- emotionally, spiritually.
Here's the quote I want to share:
"When the prophets declared the Israelites to be disobedient, they often proved it by exposing their lack of concern for the widows around them (Isaiah 1:23; 10:1-2; Ezekiel 22:6-7; Malachi 3:5). When someone cared for the widowed, it was worthy of note (Job 29:13)."