Mark actually reaches the MAV (Mars Ascent Vehicle) at the end of Chapter 24. When he reaches it, he describes it as "an impressive 27 meters tall" and "gleaming in the midday sun." When Mark then pulls up to the MAV in his rover, he stumbles out "excitedly . . . falling to the ground then scrambling to his feet." He is in such a hurry to get to the MAV and so excited that he can barely manage to keep his balance. Then, when he is back on his feet, he reaches out towards the MAV with both arms, "as if in disbelief." He has spent so long trying to get to the MAV, and now he can't quite believe that he has actually reached it. He leaps "into the air several times, arms held high with fists clenched." After celebrating ecstatically, Mark becomes "fatigued" and simply stands, "with arms akimbo," marveling at "the sleek lines of the engineering marvel before him."
From these descriptions, we can see that Mark's reaction when he finally reaches the MAV is excited, ecstatic, and awe-struck. Given that the MAV represents Mark's best chance of escaping Mars and returning to Earth, and given that he has been stranded on Mars for approximately eighteen months, Mark's reaction is, of course, entirely understandable.