Don't be that person. The person who puts a half-hearted effort into their workout and doesn't break a sweat. When you exercise, you need to show up and put in your best effort or you're just wasting your time.
First, I'll tell you some interesting facts about sweat, then I'll tell you why you should use it as an indicator of how well your workout is going.
How does sweat work?
Sweat is the body's natural cooling mechanism. When you exercise, your heart rate increases and your internal temperature rises. This stimulates the two to five million sweat glands you have distributed under your skin. Your sweat carries the heat generated by your body to the skin's surface, where it evaporates into the atmosphere, lowering overall internal temperature. Think of a bead of sweat as a vehicle for heat conducted by your body: it transports body heat to the surface, where it can diffuse into the external world. And that protects the integrity of your cells and organs from the dangers of overheating. (source: Shape.com)
Using sweat as a barometer for your workout
Of course different people have different physiologies, and some people sweat much more than others. And some people actually have conditions where their bodies cannot sweat properly. But in general, sweat is a good indication that your heart rate has been elevated for a period of time and your body is finding it necessary to employ its cooling mechanism.
For me, whether I'm resistance training or doing a cardio workout, it seems like the sweat starts kicking in at around 20 minutes. I know that if I'm not sweating, I'm not pushing myself enough.
Sweat is your body working to cool itself, burning calories and fat in the process.
I see many different types of people at the gym each morning. Some are having a great workout. Some are strolling around the track. Some are playing with their phones, reading a book or a magazine while they leisurely use an incline bike or an elliptical machine. I've actually seen women at the gym who obviously have showered, done their hair and put on their makeup already. Are you kidding? Even in the weight room, to often I see people who are lifting weights that aren't really taxing their muscles.
Oh, but they don't want big muscles. They just want to be toned. I have news for them, there is no hazard that they'll ever grow muscles at the rate they are going, or even tone the small muscles they have. They are burning a few calories, period. They are wasting their time.
I've seen these people day after day, week after week, year after year. Kudos to them for having the discipline to show up each day, but in the same that I was losing 75 pounds of fat, they didn't lose a pound. They are still fat. They didn't show up. They didn't exert themselves. They didn't get the results they wanted from the work they didn't do. How sad.
I'm not saying you should overdo it and hurt yourself, but typically there is more chance of under-exertion than over-exertion. Listen to your body - you want to break a sweat, you want to push yourself in order to gain the rewards of a good workout. If you're into walking, great! Pick up the pace for a bit and work up a sweat. Pick up the pace when you do any exercise.
When you work out, show up and get the most benefit out of the time you put into it. You owe that to yourself.