No, it's not necessarily wishful thinking, because we were not meant for this world alone. When God created us He put part of Himself within us -- our soul or spirit -- and just as God cannot stop existing, neither can we, even if our bodies die.
This is why down inside we all sense that death is not the end, but that there must be something beyond the grave. We can suppress that feeling or even deny it -- but it's still there, even when we don't have any apparent reason for believing it. Why do we have this hope within us? The Bible says it is because God has put it there. The Bible says that God "has also set eternity in the hearts of men" (Ecclesiastes 3:11).
Believing in life after death isn't just guesswork, nor is it simply wishful thinking. We know there is life after death for one reason: Jesus Christ, the Son of God, died and was placed in the tomb -- but on the third day came back to life. By His resurrection Jesus showed us beyond all doubt that there is life beyond the grave, and that by believing in Him we can be with Him in heaven forever.
Don't gamble with your soul, and don't risk ending up in that place of absolute despair the Bible calls hell. Instead, commit your life to Jesus Christ and trust Him alone for your salvation. His promise is true: "I am the resurrection and the life. He who believes in me will live, even though he dies" (John 11:25).