Many people believe that men invented gods because human beings are the only animal that can anticipate problems in the relatively distant future and worry about them. Since humans can't control things like the weather, floods, other natural disasters, sudden military raids, fertility, etc., they made up gods to control these things, then made up ways to control the gods through offerings, contracts, and the like. Most likely, the first prayers were about simply asking the clouds or sky to bring rain, or the wind to stop, or the sun to become warm again. From there, the wind, rain, sun, etc. became gods themselves, and in many cases, were later given anthropomorphic shapes (but not in all cases).
As time progressed, the gods of many pantheons became able to grant favors across a wide spectrum of human endeavor. The issue of asking a god to do a favor, then sacrificing to that god, and not having the favor happen could be attributed to many factors, among them a too-small offering, an incomplete or inaccurate recitation of a ritual, interference from another god whom one has offended, or direct opposition from a different pantheon of gods, for instance, in losing a war.
Monotheism was a brilliant invention that simplified all this into "one-stop shopping." As before, the single, one, true God will protect and provide for his worshipers if the rules are followed, and usually if the rules are followed by all in a given group.
Some wrong information in the answers, above:
1. It is unlikely that Zeus was derived from the Sumerians. He corresponds more closely to other Indo-European sky gods, and was probably brought from whatever land developed proto-Indo-European. It is quite certain that, as societies intermixed, they borrowed myths from each other and attributed those myths to their own gods. It is also quite certain that ancient peoples came to see gods of other cultures as their own gods under different names. But it is not at all certain, or even likely, that all gods originate from Sumer.
2. Homer did not create Greek mythology, though we owe him much for recounting so many myths and gods that would have been lost to us, today, had his poetry not been written down. Homer was clearly working from a much older, oral tradition. Not only does his poetry contain Mnemonic Devices to aid the singer working from memory, but it also contains references to things (like boar's tooth helmets) that didn't exist in his time, but existed earlier. In addition, when Homer breaks strict meter, it is often because the words he is using dropped the "w" at some point, and used to fit the meter, but didn't any longer.
Homer is writing about the Mycenaean period in Greek history, and there are Linear B tablets from that period enumerating some of the gods about which Homer wrote.
3. Troy has quite definitely been found, and it is where Homer said it would be, and his description is remarkably accurate, right down to the location of springs. Troy is at a site called "Hisarlik" in Turkey, on the eastern side of the Dardenelles, just east of the island of Bozcaada.
4. Egypt may have had some influence on very early Greeks, but the Greek gods bear little resemblance to the Egyptian ones, being Indo-European in flavor. Most Greek philosophers did not learn from the Egyptians, who had nothing like the variety of philosophies extant in the Periclean Era and beyond.
5. Every god was and is made up by people. There is no such thing in reality. Get over it already.
When men still worshiped ugly idols, there lived in the land of Greece a folk of shepherds and herdsmen who cherished light and beauty. They did nto wordhip dark idols like their neighbors, but created instead their own beautiful, radiant gods.
Well in ancient Greece they had very mysterious acts of nature like lighting or dying so storytellers and priests say it was a man named Zeus throwing lightening or when we die hades gets just for there own peace of mind and they also made gods for things that were popular and great influence on the greek way of life and the story of posedin was made so when sailing the sailor had the act of knowing someone was watching them and for battle the soldiers thought ares was with them so see gods were just things to live by and ease the mind
Yes, there are stories were the Greek gods come down to a mortal. Usually this happened because the mortal angered the gods.
Greek Mythology does not come with a start date.
They were born of older primeval gods and goddesses of creation.
The Greek gods did not compete for or qualify for their qualities, they just had them from the start.
Zeus was the ruler of the Greek Gods and Kronos was the ruler of the Titans.
the acient greek gods
Yes, there are stories were the Greek gods come down to a mortal. Usually this happened because the mortal angered the gods.
the Greeks belived that the gods were born in mount olyimpus
They come from beliefs of kings or rulers
Greek Mythology does not come with a start date.
They were born of older primeval gods and goddesses of creation.
Yes.
Greek, the origin of the word "pan" (all) and "theon" (gods) is Greek too.
The Greek gods did not compete for or qualify for their qualities, they just had them from the start.
They were replaced by other religions: Christianity, Islam.
Nectar is another name for ambrosia, the food of the gods (in Greek mythology) which when eaten grants immortality.
No, the Greek gods are myth.