Part of Venoms appeal is that his alien suit offers him a whole host of powers. Beyond that, however, there are factors within him that can augment that power to gargantuan proportions or diminish it entirely. One of these main components is a hatred for Spider-Man, something the symbiote feels after having been rejected by him - it originally sought out Eddie Brock at the church primarily because it could sense his similar primal hatred of Spider-Man.
Conversely, when Spider-Man first fought Mac Gargan, the former Scorpion turned Venom, he defeated him handily. This could speak to his inexperience with the suit, but Spider-Man surmises something different. He thinks that Venom wasnt as strong in this instance because Gargan didnt hate Spider-Man as much as Brock did.
Many of Spider-Mans foes have personal reasons for hating him, but only Venom can, in a tangible way, draw power from this emotion. Considering the close ties between Venom and Spider-Man, this is a unique concept that really suits the character and strengthens his villainy.