When you are a business man, you can claim all reasonable expenses incurred in connection with your business. As a salaried person, such expenses cannot be claimed. It is because your salary income will be computed by giving exemptions for HRA, LTA, PF and valuation of perquisites like company leased accommodation, conveyance, medical expenses. All these expenses are personal in nature but you still get benefit of exemption / deduction.
It is the responsibility of the employer to compute your tax liability and deduct tax accordingly. In this process, he can consider other incomes, apart from salary, as declared by you, except loss from business, capital gains etc. He can also consider the investments made by you and allow deductions u/s 80C etc. The employer thus acts more or less like a representative of Income Tax Department to determine your tax liability and deduct it from your salary. Hence, the filing of tax returns for salaried persons is normally simple.