Why is tcp reliable and udp unreliable protocol?

TCP is a connection oriented protocol. This means that it expects the target host (e.g. computer) to acknowledge that a communication session has been established. Most commonly used protocols, (e.g. http, ftp, telnet) use TCP. UDP is a connectionless protocol. The initiating host does not care whether the destination host receives the packets or not. This is by design. Two good uses of this are "syslogging" (system logging) and streaming audio. A syslog message might be sent, and the sending host doesn't care if the message makes it or not. It just sends it. If the target host gets it, fine. If not, fine.