Of course there are chances that other people might be able to steal your bitcoin. Always be careful on the websites that requires private information, especially those who asks for your private key or password, 99 percent of them are fraud or fake.
Good advice! Take this sentence as a proverb ; "Do not give your private key even to your father". It is very important, if someone get your private key then you are vulnerable to all kind of threats. Keep it safe, use 2 level verification and use hardware wallet if possible.
Although it is possible, its not probable at all. I mean so many combinations can be framed. The Bitcoin private key has 64 symbols, now just go into some permutations and combinations. implying there are millions possible private key combinations. If it was so simple like just developing software, many wallets would have been hacked as of now. In a span of 9 years that has never happened, and never will. Brute force on bitcoin wallets is just like a dream with the current processors working at maximum rate.