What is the correct meaning of having a good opinion of Allah, and does a person's belief that a certain matter will not occur, such as getting married, contradict it, or does having a good opinion mean always believing that good will happen, such as marriage, a job, or success?

A Muslim's good expectation of his Lord, that He will grant him success in marriage, is a part of having good expectation of Allah. He must also take the necessary steps, such as supplication. It is not necessarily considered a bad expectation of Allah if he thinks he will not be successful in marriage; his fear might stem from his sins, as Allah Almighty says: "And whatever strikes you of disaster - it is because of what your hands have earned, and He pardons much." And as the Prophet, peace and blessings be upon him, said: "Indeed, a servant is deprived of provision because of a sin he commits." And marriage is a provision that a person may be deprived of due to his sins.