Luke 14:33 In the same way, any of you who does not give up everything he has CANNOT be my disciple.
"Cannot" in the above verses… I’ve always mentally translated it as "not allowed". If you don't carry your cross and if you don't give up everything you will not be allowed to be Jesus' disciple. That is true. He won't let you half-heartedly think you are following him, but there's more to it than that.
Literally, the word "cannot" is "not able". If you don’t deny yourself you are not able to be Jesus’ disciple. You won’t make it. You are not able to follow "in his steps" if you don't deny yourself.
As an example, consider a firefigher's gear:
To be a firefighter, you MUST have very special equipment. A firefighter's jacked and pants is made of a fire resistant material called Nomex. He wears a tank and facemask so that he can have a fresh supply of oxygen in a smoke-contaminated environment. The helmet is made of heat resistant, heavy-duty plastic that protects the firefighter from many types of danger.
So... Anyone who does not wear his Nomex jacket and pants CANNOT be a firefighter. Unless you take up your tank and mask you CANNOT be a firefigher. They will not allow you to be a firefighter AND you would never last as a firefighter. If you tried to be a firefighter you would get burned up; you are not able to do it.
To be a disciple of Jesus you must "carry your cross" and "forsake all". You must lose your life in his in order to find it (Matt. 10:39). These are not just random requirements that Jesus is saying. Jesus wants us to be successful; he died so that we would be successful--so that we would triumph over the devil through Him! He wants us to be his disciple and so he says that we need to carry our cross, we need to deny ourselves, etc. It will be too "hard" for you, if you don't. You won’t be able to stand up against the schemes of the devil. Temptations will prove to be too much. You’ll fail. You won’t last as his disciple. It is impossible—YOU CAN’T DO IT!
And so, Jesus wisely and lovingly has told us what to do so that we CAN be his disciples! Everyone who wants to be a firefighter must take up a firefighter's equipment or else they will get burned up. Everyone who wants to be a follower of Jesus must take up his cross and deny himself or else you'll get defeated by the enemy. "You CANNOT be my disciple."
By the way, you cannot be a "Christian" without being a "disciple". "And the disciples were first called Christians in Antioch" (Acts 11:26).