There are several reasons the UK may legitmately leave the EU. It is a major sticking point in current politics within the UK right now. A brief list of some of the reasons and the pro/cons for each is given, but it is my no means a comprehensive listing.
Trade - The EU provides important trade benefits with member nations. A negotiating bloc is formed and allows for lower costs of goods. However, this restricts the UK from forming trade with non-member countries without overcoming difficult regulations. Outside the EU, the UK would be able to maintain trade with member countries on individual trade relations and re-establish trade outside of the EU with other developing countries which are hampered by current EU trade regulations.
Jobs - Much like trade, the EU allows easy access for workers throughout the member nations if they are willing to travel. The downside is it makes it difficult for the UK to control its borders due to the open flow concept the EU demands. Jobs might be limited, but internal security could be increased which is a concern since the rise in terror incidents.
Regulations - The EU has many regulations which hamper economies. These regulations also ensure fair trade among member nations and protects the soverign rights of member nations from encroachment from others. Without the EU regulations, the EK would be able to adjust working hours, open resource development and regulate industry to meet the UK needs specifically rather than the EU in general.
These are not the only reasons, but all revolve around one central issue for the UK; the UK is seeking more autonomy in the regulation of the country.