One's presence over internet, as an individual or a business, is in part defined by the domains he or she is using to post articles, products and/or services. An appropriate domain name will boost your traffic and your popularity, it may become a trademark if it wasn't already before you have started the online activity. So you have to consider your domain name as assets you must protect and to improve their values in time.
There are a lot of possible reasons people may loose their online domains, some are very common, some are not so often related with every day practice.
You can lose visitors and customers because of typo domain names
As I've written some time ago on a websited related with web design, you should reserve different but similar domain names for your business. The reason is simple: when information is being transmited from one person to another there can always appear errors, people may mispell or type with errors the name of your site. If the domain name have digits or words separated by hyphens reserve a domain name similar: with the same words, but no hyphens and with the digits as letters.
It does not look that you are protecting the domain name in this way, but in order you have one famous domain with a great working website you must cover all the field, not to let anyone take advantage by your work and ideas.
You can lose your domain name if you delay to make or renew the registration
Some TLD allow you to register your domain name for life, so you don't have the problem with the renewal. But if you decide to register a name for you domain and you delay it too much some one else can do it first. The same thing can happen if your domain name expires and you delay the renewal. It does not cost much, but you have to protect you domains: no website, no business. No business, no money, no reputation.
A few years ago when high profile movies where annouced at Hollywood some people did hurry to registrate the domain names for those movies and they tried to sell them later to the production companies. But lately the production companies learned from their mistakes and started reserving the domain names before announcing the movies.
Use privacy when register or renew a domain name
The whois records can be accessed by everyone that knows where to search. When registering or renew a domain you have the option to let the information public (and everybody will see them) or private (and only you and the registrar will know about the domain owner identity). The difference in price is only a few american dollars (the privacy costs). If you let the information public every hacker or bad person can contact you or can try to impersonate you in order to take your domain.
If your domain is just a startup or not so important you may not care so much, but when you put a lot of energy and money in the development of your website you may be hurt.
Safeguard your domain name against theft
Also during the registrering or renewal you can have registrar lock activated. This will lock the domain name making it impossible to be stolen unless you log into your account and unlock it. It can happen that hackers can hijack an unprotected domain without the owner's knowledge, disrupting the website.
Protect the access to your registar with a strong password and ensure your full control over your domain name
The most direct way to hijack and misuse a domain name is to access the registar and modify the registration account. It's not easy as the registering domains are protected against attacks, but you consider your own protection against such things: use a strong password to protect the access to your registar. For that you can include numbers and special characters, lower and upper characters in the password.
In some cases (depending of the TLD), the registrant and the admin contact can make changes to the ownership information for a domain name. Under some circumstances the technical contact may also be able to make some changes to the information if the admin does not respond questioning a requested change.