The lights, whistles, bells and sirens tend to grow old very fast and the "adrenaline" wears off (if they are really into their job) after a few calls.
I just wanted to second this statement! One of the best parts of every run for me is marking onscene and being able to turn off that darn siren. It just doesn't save that much time to use it and the lights, and I would rather not go anywhere emergent if I didn't have to do so.
The reason you go into this job may be different for everyone, but for me it is because of runs like the one we had earlier this evening. We were called for a sick person and show up to find an older lady in the bathtub with an altered mental status. She was a known diabetic, so we checked her blood sugar - 35. She was able to talk, albeit, she was very confused and scared. We got some orange juice in her, and some oral glucose and rechecked her sugar a little bit later (aprox 15 min). It had went down to 27. We called for additional help to get her out of the tub and down the stairs. While we waited, I sat in the bathroom on the edge of the tub just holding her hand and talking to her. Eventually, she calmed down (at one point she was crying). We got her onto our cot and transported to the hospital. By the time we got there, she was coming back around - the sugar had finally gotten into her system - and was appropriate. Before leaving the hospital, she thanked me for being so kind to her. It isn't something we expect to hear from our patients, and most of the time we don't. It isn't that most of them don't appreciate what we do - they just have other things on their minds. However, that one run every so often where you do hear it gets you through all the ones where you don't.
If you don't truly care about other people or want to do whatever you are able to at the time of their particular crisis, you don't need to be in this job. I have never had a desire to fight fire, although I do currently work for the fire department. I love the guys on my crew and wouldn't trade them for the world. It's no secret though that while they do the EMS runs, it is the fire runs that really motivate most of them.
Think about what you really expect out of a job, talk to people that do that job, and, if possible, do a ride-along. If this isn't something you have ever really been exposed to I think you may be surprised about what the job actually entails.