Does Islam prefer joy and optimism or seriousness, or should a Muslim always expect the worst?

A Muslim should have a good opinion of Allah, be optimistic, and rejoice in blessings, while diligently pursuing lawful means and seeking Allah's help. For Allah says: "I am as My servant thinks of Me. If he thinks good of Me, he will have it, and if he thinks evil of Me, he will have it." There is no harm in considering the consequences of matters and anticipating and remedying failure. Allah has encouraged His servants to rejoice in His grace and mercy, to be grateful for His blessings, and to anticipate and spread good news.