Some commentators cannot understand why Jesus places this example with the other three, as it does not seem to show having a good attitude under trial. However, having a godly attitude in parting with what we hold dear can be a test for us as well. The parallel scripture in Luke 6:30 shows that it follows the pattern of the previous illustrations: "Give to everyone who asks of you. And from him who takes away your goods do not ask them back."
Many believe that what Jesus requires here is foolish, that is, to give to everyone who asks of us and to allow our goods to be plundered without objection. Perhaps Luke 6:34-35 helps to clarify what Jesus intends:
And if you lend to those from whom you hope to receive back, what credit is that to you? For even sinners lend to sinners to receive as much back. But love your enemies, do good, and lend, hoping for nothing in return; and your reward will be great, and you will be sons of the Highest. For He is kind to the unthankful and evil.
His illustration in Matthew 5:42 deals with borrowing and lending, not with allowing oneself to be plundered. As in the other illustrations, His primary point is that it is preferable to suffer loss or harm than to retaliate or worsen the situation. When we give to someone in need, we should not expect to be repaid for our generosity, and we should certainly not take steps to force reimbursement. Christian charity should be done without expectation of gain. Yet, God sees, and He will show us favor: "He who has pity on the poor lends to the LORD, and He will pay back what he has given" (Proverbs 19:17).
If a person asks for a loan of money or goods, we should approach the request assuming that he makes it in good faith, if there are no extenuating reasons to doubt his sincerity. We should, however, keep in mind other principles from God's Word, such as being good stewards of what God has given us, taking care of our own, not encouraging laziness or sustaining the idle, not supporting vices (alcohol, drugs, or other addictions), and not being a party to shady or dubious get-rich-quick schemes. Jesus' suggestion is that, if we do lend to others, we might as well consider that money to be gone forever. The struggle to regain it will probably not be worth the effort, not to mention the damage it could do to relationships and one's character.
In short, what does His final illustration require of us? It asks of us, not only that we should lend without suspicion and with no eye to profit, but that we also should have a generous spirit of outgoing concern for a brother or sister in need.