You don't get high like you used to because you are physically addicted. Try taking a six week holiday from smoking and you will find that it will make you just as high as you like, or at least until your tolerance builds up again, which tends to happen faster each time you go back to toking.

As to pot leading you into harder drugs, the more THC you need to get the high you want, the more you are likely to get hooked on something stronger. It's the same problem: the more you use, the sooner you need more.

The more important any drug becomes in your life, the sooner you become dependent. If "Mary Jane" (marijuana) is already your best friend, naturally she is going to want to introduce you to her other friends.