"Very truly, I tell you, anyone who does not enter the sheepfold by the gate but climbs in by another way is a thief and a bandit. The one who enters by the gate is the shepherd of the sheep."
When I think about scenarios involving "climbing in by another way" than the front entrance, I think of people trying to SNEAK IN to someplace. Maybe to get in without paying (Amusement park? Sports Stadium? etc.)... or trying to get around ridiculously long entrance lines (festival? concert? fair?). But when Jesus speaks about it, he's not talking about people wanting to get IN someplace forbidden... he's speaking of people wanting to get in, in order to STEAL what's inside (art thieves, jewelry thieves, bank robbers, etc.). But more importantly, he's speaking about spiritual leaders (namely, himself!).
Sheep come in and out of the safety of their pen through the front gate. The shepherd leads them in and out himself. The sheep should know not to follow anyone who "climbs the fence" and enters by any other way (But then again, sheep aren't known to be the smartest critters on the farm!).
Jesus is warning his followers not to be deceived by "false shepherds." Don't follow anyone who doesn't come in through the front gate - open, in plain sight, with everyone knowing exactly who s/he is. Don't be led out by anyone except the Shepherd - for it will not lead to the abundant life promised.
Ultimately, the Shepherd is the one who truly cares for the well-being of the sheep. He's not trying to use the sheep or sell the sheep or even make a profit off the sheep. His job is to love, guide and protect the sheep. It's in their best interests to follow Him.
So why are we so quick to follow other "competing" leaders in our lives? Whether it's a charismatic leader... a passion in our life... a job/career... an addiction, etc... Anything that puts itself as the #1 priority in our life is ultimately a false shepherd.
Look to the front gate.
Follow the true Shepherd.
Listen only to His voice.