Were people to be given everything they claimed, some men would claim the properties and blood of (other) people. However, the onus of proof is on the plaintiff and the oath is upon the one who denies the claim (defendant)

The Prophet (may Allah's peace and blessings be upon him) pointed out that if people were to be given on the mere basis of their claims without proof or evidence, people would claim the property and blood of others. However, a plaintiff is required to produce evidence and proof for his claim. If he has no proof, the claim shall be presented to the defendant, and in case he denies it, he is required to take an oath, after which he shall be acquitted.