The verbs agree and concur both mean to have the same opinion as another. However, agree often presupposes an interpersonal process by which this harmony is reached. The context might involve discussion and negotiation (they agreed on a theme for the party), or consent or compliance (agree to a pay cut) between the parties. To concur, on the other hand, is to show accord in matters of opinion, as of minds that have independently run along the same channels (to concur in a judgment about a painting). It would be unusual to say, “I concur with you,” not only because concur is a more formal word choice than agree, but also because concurring is more impersonal: one doesn’t usually concur with a person but with the substance of an opinion or judgment.
As a simple negative of happy, the adjective unhappy has a fairly broad range of application. A person described as unhappy may be sad or miserable, or they may simply be displeased or discontented with one particular thing, as when one is unhappy with the outcome of something. The word miserable, on the other hand, leaves less room for interpretation. A miserable person is intensely unhappy, to a pitiable degree. Where miserable comes in especially useful is in describing a wretched state or condition, such as a miserable cold, a miserable pregnancy, miserable weather, miserable living conditions, or, worst of all, a miserable existence. Miserable is a more feeling word than unhappy, and carries a strong element of the pathetic. As with all things pathetic or wretched, it can evoke either pity or contempt. For example, there’s no empathy in calling someone “a miserable so and so”: for miserable here you could just as well substitute lowdown or despicable.
Something that is excellent is remarkably good. The synonym sterling means “thoroughly excellent,” but its history adds a little shine. Sterling entered English as a noun referring to the silver penny of the Norman dynasty. Over time, it came to be used to refer to British money more generally, as in "pound sterling." Along the way, sterling developed several attributive senses relating to money and more specifically, to the quality of metal used in money. This is where we get the phrase “sterling silver,” designating silver that has a fineness of 0.925, now used especially in the manufacture of table utensils and jewelry. The use of sterling as an indicator of quality was extended to things immaterial, such as a sterling reputation—to indicate that they meet or exceed the highest standard.