Firstly, Haig planned the Somme as a breakthrough battle, designed to smash through the German defences and open a path for the British army to invade Germany, advance on Berlin, and thus win the war. Haig said as much, in these words, in the lead up to the battle. Assertions that he was planning for 1917 or 'training' troops (Kitchener's Army recruits had been training for over 18 months before the Somme) are nonsense.
Secondly, Haig was overconfident. His told his subordinate commanders that the German lines would be completely destroyed by the 5 day artillery barrage preceding the attack, and all the German defenders killed. He assured them all the British soldiers would have to do was walk into the German positions and take them over. In the event, the artillery bombardment was largely ineffectual, failing even to cut the German barbed wire obstacles in many places. Further, the German defenders, far from being wiped out, sat out the bombardment in deep underground concrete bunkers (called stolen), and designed for that very purpose. Once the bombardment ended, they had plenty of time to return to their (largely intact) trenches and mow down the British infantry with machine gun fire.
At least one British general asked Haig for extra hospital trains, in expectation of high casualties, before the attack. 3 times Haig was asked, 3 times he refused.
Finally, despite the debacle on the first day of the attack - 60,000 British casualties, 20,000 of which were killed,