To answer this question, different species have different reasons. Some, like the Wooly Mammoth and many if not all of the dinosaurs' extinction, died to natural causes. The Wooly Mammoth's being rapid climate change that left it unable to adapt quickly enough, and the dinosaurs' reason is not known for sure, but most believe that was caused by an asteroid from outer space. However, most extinctions today are caused by us. Even though we may not mean to, our harvesting of resources can sometimes take strides toward different species' extinction.
Pretty much any way you look at it, the loss of habitats is our fault. We've cut down tons and tons of acres of forest habitats for paper. We are basically kicking these animals out of their homes. Not only that, but we're taking away the homes on the market. Rain forests are where over 50% of plants and animals reside according to rainforestfoundation.org. Since we cutting these down, more than 50% of the world's plants and animals are slowly losing their homes. Since ecosystems have certain limiting factors such as not enough food and space for more than a certain number of animals, some animals cannot survive with the loss of so many habitats and very little remaining. Also, to the far north and south at the poles, global warming is the cause for habitat loss for animals in need of those cold environments. According to "Climate Change: Vital Signs of the Planet" by NASA, "Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2). This happens because the coal or oil burning process combines carbon with oxygen in the air to make CO2." Global Warming was caused by us, climate change will affect all species, as they may not be able to adapt as quickly as humans. Currently, it is most significantly affecting polar bears and other arctic/antarctic animals. Their ice is melting, leaving them a harder lifestyle and it more difficult to find food. This habitat loss is most often the cause of endangerment and extinction. However, there are still more causes that are also pretty common.
Over hunting and fishing is mostly our fault, but it can be caused by other species. These other species are mainly invasive species. According to invasivespecies.gov, "an 'invasive species' is defined as a species that is: 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Invasive species can be plants, animals, and other organisms (e.g., microbes)." However, we are usually the ones who carried in the invasive species that are over hunting and fishing from other areas. According to worldwildlife.org, one of the main reasons for the critical endangerment and even extinction of several species of dolphins and porpoises was fishmen overfishing the area and leaving a very small food source for the dolphins. This also can happen when invasive species are brought in. The invasive species' food sources gains a predator and doesn't have enough time to adapt before many are wiped out. Fishermen and hunters can also overfish and/or hunt a species to extinction or endangerment.
Bycatch is the "incidental capture of non-target species such as dolphins, marine turtles and seabirds" according to worldwildlife.org. This happens during commercial fishing because of the large nets in areas populated with a lot of a certain kind of fish. The fishermen aren't the only ones who want the fish. Dolphins, porpoises, and sharks swim by to try to catch a dinner of their own. Instead, they end up caught with their dinner. Bycatch contributes to most if not all endangerment and extinction of animals in the sea. Sea Turtles have become vulnerable due to bycatch. Sea creatures like turtles get tangled in fishermen's nets and often don't make it out alive. Though bycatch is accidental, it has a very large effect.
In summary, we're the ones who drove these animals into extinction or endangerment. We left these animals with no food and/or no home, and made them fearful in areas that used to be comforting. We caused this, and were the only ones who can make the change to stop it.