It depends on what aspect of the business you are looking at..
Long hours, physically demanding job (and even if it seems easy now, will it be easy when you at 30? 40?)
Times when you will working include nights, weekends, holidays - this makes it difficult to have a social life with people who are outside of the industry. If you have a significant other (or friends and family) that have traditional 9-5 jobs, your hours are usually during the time they have off. This can put a big strain on relationships and can also make it difficult to celebrate special occasions with family members, attend weddings, etc. Also, depending on the type of kitchen you work in, the tasks can become very repetitive and monotonous.
Owner: This is a whole other level of commitment. You are constantly the go-to person for any and all problems. If you can imagine the stress people associate with being a new mother (baby constantly crying, needing attention/feeding/changing/ etc) and the fact that you can't just take a break or walk away from it while it is "new", this can be a taxing job. Especially when you consider the pressure from being so financially connected with the business. Every problem becomes more personal and stressful because if the business fails, it has taken so much of your time and money. Of course, a business can level out and get established so you can have a day or two off.. but most restaurants (when they first open) don't make a profit for 6 months- 1 year (depending on how much the start up costs are and how long it takes to establish a client base), and for the first several months you will likely be there every day, and will be receiving calls and texts during times when you aren't there.
This takes a lot of work and love. BUT- if its something you enjoy doing, and you find the challenges exciting and engaging, it can also be incredibly rewarding.