With someone carelessness, Bitcoin can be stolen. Once someone is lost with his or her private key or seed phrases,someone Bitcoin can be easily stolen. But if your private key and seed phase are only known to you and nobody gets access to them, it is very impossible for anybody to steal your bitcoin. So the important thing to do to avoid having your Bitcoin stolen is to provide proper security for your Bitcoin wallets by hiding the privacy and seed phrase wisely in a location where nobody will get access to it, unless only you or the person trust and show where you hide those your private key or seed phrase.