Maria, 
I understand you question and where you are coming from. there is a quote out there that says "I never said it would be easy, I only said it would be worth it". We keep those rules (commandments) because we believe: 
1. God has commanded them of us 
2. God will bless us if we keep them 
3. After trying God's promise to keep a commandment, we see that he does bless us immensely with happiness. Because we have tried this out for ourselves and know that it works, we will want to obey all of his commandments. 
 
I have found that I have more liberty and freedom by obeying those commandments, rather than viewing them as restrictions. No dating until 16 - I didn't have to worry about the pressures that come with having a girlfriend...and I definitely saved some money! No sex before marriage - I never had to worry about dealing with an unwanted pregnancy or sexual diseases. No coffee or caffeine - I didn't have to worry about my brain being taken over by short bursts of energy that always have a downtime waiting on the other end....plus I didn't have to ever worry about becoming addicted and having to spend $5 a day at Starbucks just to feel like I could start my day. 
 
Hope this helps

I have a good friend that is a mormon. I asked him about their rules, and he had a comment that really struck me. He said, "These walls don't keep me from having fun; They keep me from becoming miserable." 
Some things that didn't make sense in the 1800's (no drinking, smoking) make sense now. modern science shows heavy drinking/smoking is very detrimental. I don't want to get off subject, but, maybe in the future the rest of their rules will be made crystal clear.

before i give my thoughts, I will start with a story. I went to a camp where I was the only girl without a bikini. when somebody asked why my non-member friend responded that "she's mormon, she's not allowed" to which the other person responding with a unique"that sucks" end story. I thought a lot about that and I decided that I follow these rules because I have decided for myself that not only do i belive thay are right, but they are the way I want to live. But refusing to do something just because thieer are limits, are your own choices not limited. most people don't need a law to tell them not to kill, just as I don't view these as limiting, but rather standards by which I have chosen to live my life. it's okay if you don't agree with my hypothetical question, its just something i've been aching to say somewhere for a while.